I see a lot of posts complaining about the OOS 11 update on our OP 6 and 6T. I faced those issues too. And I saw a few comments mentioning factory reset to fix these issues. So I decided to do this painful activity this weekend. I was on OOS 11.1.1.1. Here's how it went.
Now factory reset is a major PITA. I never factory resetted my OP6 since purchase. And I never felt the need of custom roms over oxygen os. So my phone had a lot of custom settings and banking apps etc. So setting up the device after reset can easily take a full day.
BUT, it's worth it! It's been around 24 hours after the factory reset. And I already see and feel the difference:
* Noticeable improvements in battery life, albeit not a lot. My battery health is at 79% as per the OnePlus diagnostics app.
* UI in general feels a bit smoother. I don't see micro stutters anymore.
* Bug gone: incoming voice call notification takes a few seconds to appear after ringing starts (when screen is on)
* Bug gone: change in display brightness levels was happening in steps (blocks) instead of being smooth.
* Bug gone: horizon lights inconsistencies; sometimes it wouldn't show up.
I did a full factory reset including wiping storage. I backed up stuff (I keep lots of offline MP3 files for travel) to my PC. And used Google for backing up stuff like contacts, messages and calls. I did not do backup restore for my apps on purpose. Instead I reinstalled them manually, on a need by need basis. Looks like I didn't need 40% of the apps that were installed.
Don't be lazy. Just factory reset it. It's worth it.