Recently, I got hit with the 7.3.5 lag bug. During raids and combat, server times went up to 9200 ms, and it took around 35-40 seconds to mount after clicking the mount button. If you're experiencing this, you can try to fix the issues the same way I did. I hope it helps.
- Rename all 3 of these folders: Interface, Cache, and WTF. Don't do one and see if it fixes the issue: rename them all at once. I tend to put _bak at the end of them, so I know what they were originally.
- Open the launcher, select the Options gear right under the Legion icon in the center of the screen, and click the Scan and Repair option.
- Get into game, lament that all your configurations have to be redone.
- Test out the game, verify that your issues are fixed.
- Add in your addons one-by-one, testing after each one.
- If you encounter lag again, note the addon causing the issue and respond to this post with that addon, so others will know what causes issues. Also, DELETE ALL 3 FOLDERS and re-install the other addons again.
I can't stress it enough: if you get the lag, you can't delete 1 or 2 of those folders. It seems that Blizzard chose to affect the WTF character saves with some addon pollution, and it's enough to cause the lag to persist, even after deleting the addon that's causing the issue.
Feel free to comment on anything else that you might have found to solve the issue. Happy Gaming!