McMurphey: There can be countless reasons for this. I would set every option to its default setting as a start. If it doesn't help, maybe reinstall the game. Without knowing what your computer is doing, we can only took wild guesses here. Maybe your anti-virus software is taking too much from your CPU, throttling GPU/CPU because of thermal reasons or the RAM on your graphics card is full. The RAM thing is the main reason for the crippling slowdowns I encounter after playing for a couple of hours. I have to restard the game when it occurs.
Other people have similar issues since 1.5:
https://www.gog.com/forum/cyberpunk_2077/fps_in_15 I can't compare since I started to play after the v1.5 update.
markiebaby: I've optimised using NVIDIA, the CPU usage is 11%, game is set as allowed on antivirus - its still at 11FPS when it was running fine (even with the 1.5 patch). GPU cache should clear on shutdown, and I've done that many times since this issue started.
arugulaKhan: What kind of a hard drive do you have the game on?
markiebaby: 2TB SSD - it was working fine 2 weeks ago
Eh, got me stumped :P
My first guess was Windows 7 (I was on 7 and performance took a huge dip after 1.5 was released).
Second guess was hard-drive (I had it on a regular disk and it's been night and day since upgrading to a simple SSD).
So, with those out of the way, I'm gonna say "background processes". Find one of those "turn off everything unnecessary" guides online, save it locally, disconnect Win10 from teh internets and start turning things off one by one.
Oh, and turn off antivirus protection. I know they say it shouldn't interfere with the running of things, but, well, it usually does.
Give that a shot. I run my game on an offline Win10 SSD and it runs fine.