After season 21 update game is unplayable due to every second FPS drop

Same here!

FPS drops randomly to the bottom followed with lag & stuttering

Inputdelay warning icon appears.

Since Season 21 release ...

I see these icons:

So You have

- UCMD Delay

- Lag Compensation error

Explanations here: 

https://www.dexerto.com/apex-legends/all-apex-legends-new-lag-symbols-what-they-mean-1861524/

INCORRECT! Where did you see

- Lag Compensation error icon?

I see -

UCMD Delay

and 

Prediction Error

https://afkgaming.com/esports/guide/all-apex-legends-connection-symbols-explained

In Your video 0:37.

Every single icon showed up once, how is this helping me?

Having this exact same error. Infuriating, it showed up immediately after the Season 21 update, was perfectly fine before the update. Everything else runs normally, only Apex has an issue.

The fixes I've attempted:

- using controller instead of a mouse somewhat fixed it. too bad I don't play with controller, so the hunt for a fix continues.

- changed mouse port
- changed logitech gaming software mouse illumination settings (G502 mouse, changing this fixed this issue for some people in the past. It didn't work for me.)
- got a brand new mouse, different brand, also didn't work
- restart
- reinstall
- close all other applications
- fresh install and play using a different drive
- update all drivers known to man and god

- reset graphics driver
- update windows

- changed servers
- changed mouse polling rate (apparently fixed it for some people? not me though)
- different networks (4 of them)
- cooler environment (confirmed it was not an additional overheating issue)
- removal of additional display
- steam settings modification (launch codes)
- changing the fps of the display using nvidea controlpanel

- nerfed antivirus

- modified a ton of ingame settings (did nothing):
* vsync changes
* nvidia reflex off/on
* all graphics settings to as low as possible

I already put in days trying to figure this out, but nothing had worked. Please let me know if a fix is found. It would be irritating to have to also try to modify the BIOS. It's clearly an issue with Apex, not the machine.

Found a fix that worked for me!

Performed a clean boot. EA actually has a guide for it: 

https://help.ea.com/en/help/faq/how-to-clean-boot-your-pc/ 

Let me know if it helps you as well!

Well what caused it for you? What did you enable after the boot?

To my surprise it was a password manager, I had no problem running both befor but after update I have to close down password manager to be able to play Apex 

@Conpagandawhat did you do after the boot?