Reloading causing FPS to drop to 14-25 for about 20 seconds - and saved edits in edit mode reset (particularly quest tracking)
Disabled all addons. All were to do date, regardless did so.
Logged out, did a scan and repair, then it said update (WoW, Battle.net.) Did so.
Problem was fixed for roughly 30 minutes - started up again.
Used a bug catcher - only errors showing are Blizzard LUA.
Thanks.
Likely need a UI reset, including default CVars. Many addons made changes to server-side variables that are still lingering.
To discuss this further, use the Guides > UI and Macro forum.
I have the same problem, did you find any solution?
No, sorry 
I started noticing today. Did the link that MVP posted give u any help OP?
Been having this as well. It’s definitely a new issue to 12.0. I’m not interested in doing a full UI reset since it’ll take a decent amount of time to review all the client variables I want to maintain (or, worse, track down offending ones if any of those are causing the problem). It’d be nice if there was another clear way to discover the source.
Someone in a thread there says it’s something to do with addons being loaded. I find that hard to believe though unless Blizzard changed how things are handled on a /reload. Prior to 12.0, the game would still fully load all of your addons before it actually finished the load screen and showed you the game world again. So, the only way that could be the issue is if they for some reason changed the reload logic to only do addons after the initial base UI is reloaded. But that still doesn’t quite check out, because otherwise visual elements of addons would be blinking in one-by-one if they weren’t yet loaded or functional, and nearly all of those addons require their base code to have been read for them to display functional visuals.
I’m gonna poke around the logs folder and see if there’s anything interesting in it but I’m not holding my breath.
Also having this problem specifically since the prepatch dropped. Anyone found any solutions?
It could just be luck, but I dropped the in-game foreground FPS limiter and switched to driver-based after reading the below post, and I have yet to see the issue again after a /reload. Still need to play some more but I would usually catch it on the first reload of every session.
EDIT: Still ended up getting the issue after doing a reload following a few pulls on Mythic Gallywix.
I’m having this issue with every addon disabled and the stock Blizzard UI.
I also completely reset my Cvars because I have a backup to go back to my old settings just to test, did not change anything. So I would actually suggest for people to not wipe their Cvars unless they have a proper backup.
Disabling them isn’t sufficient troubleshooting. They need to be removed from the directory, then start game, reset Cvars, then restart. You sort of went all over with the steps and then said they don’t work… 
Oh, we still blaming addons for these issues…? 
Resetting the UI, and the advanced CVAR would not technically be blaming addons, but a basic start to trouble shoot a local issue. Even the base UI can become corrupted causing issues.
This has been plaguing me as well. What I did discover is that it might not be related to addons, or perhaps not. My current observation and test are all linked to the Combat log. On a fresh “log in” and a empty combat log I experience no log on log in or reload. After a few combat activity like battleground, my Ui reloads are having this issue. This is with my addon activated and also without any addon activated.
Also, if my combat log is decently populated, switching my my chat windows between General and Combat Log generate the massive lag spike where I go from high 100fps to like 10-20 until the Combat Log is finish loading.
It’s Blizzard’s problem and has nothing to do with your addons. It generates all your combat data on reloadingUI (does the same when clicking on the Combat Log chat box). Probably due to the new damage meter. The only way to clear the logs is to logout of the game, which will fix the issue until you accumulate enough to lag again.
I don’t know which is worse, this or the default nameplates making 10,000 function calls in a matter of seconds.