How did I do it? I used to profiling addons called "AddOns CPU Usage" and "Addon Usage" in hundreds of real raid boss fights and comparing the hard data. Once the data was collected, the information was put into real use and monitored by many different people.
Here I will list the addon winners so you can add that extra addon functionality while putting WoW on a guilt free diet. Most people have helped with this have seen increases of 10-20 FPS in their game.
NOTE: In no way do I intend to show any disrespect to any addon authors who freely publish their work for the benefit of the community. I would like to personally thank each author for their work, creativity, and donations to help others have more fun in their game play. Also, if you have any comments, please keep it respectful. Countless hours of work have gone into this review, and I am only sharing it to help others who's computers might not be the latest beast of monster boxes.
NOTE: There is a difference between RAM and CPU. RAM refers to your memory. If you are not using all of your memory (which most probably are not), RAM doesn't matter. RAM usage will not effect your game or your FPS. Furthermore, the amount of RAM that an addon uses is nearly insignificant compared to your total amount of RAM inside your computer. CPU is something different! That's your processor. That does matter and does effect your FPS. This is where the focus will be.
1. DPS Meter
"Recount" has limited features, a dated interface, a single window, and it's a CPU hog. This addon set the standard for DPS meters, but some newer competitors have joined the ranks.
"Details!" is by far the best DPS meter out. It is incredibly low on CPU usage and has more features than any other meter on Curse. Everyone who uses it loves it and the details it gives are remarkable. Want a separate Healing and DPS window? No problem! Have an many as you want. It even has a built in threat meter like Omen. The amount of CPU used is so low, that I'm not sure how they are doing it.
There is only one issue, it takes a lot of RAM - which won't affect your FPS for most, but for those with too little memory who page out to the hard drive when running WoW, use "Skada". It still blows away others in performance and features.
2. Boss Mods
"Deadly Boss Mods" is another serious user of the CPU. It can extend your loading time by up to 10 seconds when going into raids or dungeons. It's configuration file alone can be over 20 megs.
Use "BigWigs Bossmods". It does the same thing and barely makes a dent in your CPU or loading time. It even uses less CPU than "Details!". It's also updated with new content faster than DBM is. There's a reason why all the world first guilds use it.
If you install "BigWigs Bossmods", you will also need:
LittleWigs (dungeons module)
BigWigs_AutoReply (if you want to tell others when you are fighting a boss)
BigWigs_CommonAuras (make sure to check out it's configuration to turn on these messages)
BigWigs_Voice (it will talk to you and tell you when important events happen)
If you'd Like additional Voices to go with BigWigs_Voice, bigwigs_voice_heroesofthestorm to add the HOTS voices, and bigwigs_voice_overwatch For overwatch.
And if you run older content like Time Walking:
BigWigs_Classic (vanilla support)
Another addon to check out is "Vocal Raid Assistant". It will tell you important things like when someone hits Time Warp or a big AoE Heal cooldown goes off. Again, this uses almost no CPU.
Look, I know that "Bartender" is the most common and does have more features, but it does take a toll on your CPU... more than I expected.
Use "Dominos". It takes almost no CPU at all and has most of the functionalty of "Bartender". How does it do it? It re-uses the Blizzard toolbars that are already in the game and expands on their functionalty. Any code that runs in native C++ over LUA script addon code is going to run light years faster.
4. Bag Addons
I tried all of them. When I say all of them... I mean it. I tried every bag addon there is on Curse and other sites and checked their features, CPU usage, and functionality.
Use "AdiBags". And while you are at it, install "AdiBags Legion" too for filtering. It's the best out, has the best features, can separate out your gear sets, and uses very little resources.
"ArkInventory" is a close runner up but does take more CPU and more configuration than "AdiBags".
5. Auras, Buffs, Debuffs, Cooldowns, and Spell Alerts
Look, "Weak Auras" is amazing when it comes to features... and if you have a super beast of a machine, it probably won't matter that much.
It's terrible on your CPU. I mean, it's a hog. The more you load into it, the more CPU it takes... a lot more. Tons more.
Try using "TellMeWhen". While this is also somewhat of a CPU user, it uses about half that of "Weak Auras". It doesn't have all the extra bells and whistles like "Weak Auras" has, but it does the same thing and is a lot easier to use and set up.
If you really want to minimize your CPU, use "Aura Frames". This addon actually writes its own internal code on the fly for the game to make it's CPU usage nearly unnoticable! It's lightning fast. Actually, it's unreal and amazing. From the author:
"The "engine" behind the screens is the LibAura that is specifically written for AuraFrames that provides aura information over all the units and even more. It also include information over internal cooldowns, spell cooldowns, totems, weapon enchantments, testing units and more. The filtering, ordering and coloring is unique. No addon has ever approached Aura Frames with the way the internal system is working. The most important difference is that Aura Frames is not checking realtime aura's on all properties but is generating real LUA code blocks based on filtering, ordering and coloring that are parsed by WoW and are executed as native LUA functions (for non nerds: its fast)!"
Why did I list it second? Because it's originally inteaded as a buffs/debuffs replacement like "Elkano's BuffBars," "Raven," "PhanxBuffs," or "CT_BuffMod". And while it beats the compition with a two handed epic sword, it also can tell you when spells are on cooldown or when important buffs proc. It just takes some time configuring it - which, in all honesty, only needs to be done once.
6. Transmog Addons
Mogit is awesome. It's even load on demand. But do you need all of that all the time?
Consider using "AppearanceTooltip". It has an impressive list of features, will show what gear looks like in the tooltips, and will tell you if you have the transmog collected.
"Legion Wardrobe" is another winner. It does more than I can list here... namely changing the appearances tab and filtering. It is super light on your CPU. The two can be used together.
7. Floating Combat Text
MikScrollingBattleText is the best out right now. Compared to all of the tested combat text addons, it has the lightest CPU usage and the most functionality.
It can even show you what you just looted (love that) and if you got any profession skill ups.
8. Alts Tracking
Oh man... you have no idea how bad the CPU usage is in most of these addons that track your alts and their banks and bags. The most popular addons in this category like "Altoholic" are the worst on your CPU.
I found only one that uses almost no CPU and is not a hog on load times. That addon is "BankItems".
Use "BankItems". It's amazing. Now you can hover over that crafting item and know that your druid alt has 12 in the bank and 2 in it's bags.
The downside is that it will not track your other characters professions or achievements. But do you really need that? Increased loading time means loading a config file the size of Texas.
9. Raid Frames
"Grid2" and "Clique" are BY FAR (like 1/10th the CPU!) the winners in the CPU and functionalty contest. I had no idea that "Grid2" was that good or that flexible. Don't be confused by "Grid" which is not the same as "Grid2" and takes about double the CPU.
Where did the other frame addons fall? Well... Healbot uses a truck load of CPU (much more than "Grid") and Vuhdo is even worse. Both of these addons are amazing... but come at a cost. If you can switch, "Grid2" even has that AoE healing predictor. To be fair, all of the ranged stuff in raids will probably be broken by Legion's raid range addon nerf.
10. Unit Frames
"Shadowed Unit Frames" took up the least amount of CPU in my testing. It is also rich in features and stable with frequent updates.
Of course, it depends what you turn on and off in the options on how much CPU it uses. Still, it out performed it's competitors in my testing.
On a side note, it uses even less CPU if you just use the default frames and move them using addons like "Leatrix Plus" (see below).
Honorable mentions that use little or no CPU
"Leatrix Plus" uses nearly zero CPU and allows you to move your frames around, auto-sell, auto-repair, get rid of map fog, and about 100 other things. I highly recommend this one. It's made by the same people that created Leatrix Latency Fix used by scores of raiders over the years.
"AdvancedInterfaceOptions" lets you configure options that were taken away in the Legion pre-patch.
"Auctionator" is the best auction house addon out. If you are using "Auctioneer", you need to get with the times.
"Bazooka" was the winner of all data broker toolbars in CPU usage and features. Just remember that the individual data broker addons use their own CPU.
"BlizzMove" lets you move any window in WoW.
"EnhancedMenu" extends the popup menu with things like guild invite, copy name, send who, armory URL, etc.
"Exorsus Raid Tools" uses very little CPU and is probably one of the best addons for raiding and raid leaders.
"FriendsShare Resurrection" is the CPU winner out of addons that sync your friends list across all your characters.
"GTFO" should be built into the game. It alerts you when standing in stuff that kills you - even if you can't see it.
"HandyNotes" takes minimal CPU and adds treasures and other items to your map.
"KuiNamePlates" was, hands down, the best name plates addon. "Tidy Plates" used a large amount of CPU and didn't offer that much more functionality.
"Macro Toolkit" - if you like macros... you should use this. You can even send macros to other players.
"New Openables" adds a popup button for opening containers (like clams) or shows quest items.
"Pawn" can show you what gear is an upgrade and uses nearly nothing of your CPU. Note that Pawn can be inaccurate for min/max end game raiding but it's great for leveling.
"Postal" is the must-have mailbox addon.
"Quartz" is a casting bar addon that reuses the Blizzard casting bars and takes very little CPU.
"Scrap" is a powerful junk seller at merchants. "Scrap Cleaner" works with "Scrap" to delete lowest cost items out of your bags when they are full - handy when farming old raids.
"Talent Set Manager" wins when it comes to changing talents with one click.
"Tekticles" will change your default fonts and make things much more readable. It will also make you laugh when you say the name.
"The Undermine Journal" uses almost no CPU but does use a bit of RAM. It shows realm-specific pricing information in item tooltips for all US and EU realms and is updated twice each week.
"TipTac" is the winner in tooltip addons for both CPU, functionality and stability.
Extras and Warnings
As a final note, there are three extremely popular addons that I have found time and time again to take up the most CPU and should be avoided or only turned on when needed.
As cool as "Overachiever" is, it's one of the worst offenders in CPU usage. This addon alone can lower your frames per second by 10-20 FPS! And this issue isn't anything new. It's been reported for years now - before Mysts of Pandaria was launched. Do you just need a search? Use "Achievement Search" on Curse. It works great. And if you need the other functionality, you should disable Overachiever when you aren't using it.
"OmniCC" beats all the button cooldown addons in CPU usage and in functionality, but it still takes a bit CPU due to the font rendering on every single button. Your only option here is to use the default built in WoW cooldowns or deal with it's CPU usage.
"Tomtom" and "Masque" can use an obscene amount of CPU and it's not really their fault. They are similar to libraries that other addons can call... but when not called properly, can ruin your FPS. Be cautious of them. They were on the top 5 most CPU usage of all tested addons.
An amazing alternative to "Masque" that uses ZERO CPU and ZERO RAM (you read that right) is "Clean Icons - Thin" found on WoW Interface. It changes all the buttons - even buttons in other addons. Do an internet search for them. You will have to manually install it. It's worth it.
And if you want to skin your interface without any CPU or RAM overhead, do a search for "MiirGui Texture Pack [Grey]" or "MiirGui Texture Pack [Blue]". They look much like ElvUI.
To change your fonts without addons, head on over to vranx.com/font.htm for instuctions.