The 'Guardian Games' update absolutely tanked FPS.
I have a 1660 Super that could run most content at 70-120FPS @ 1440p settings at a mix of highest-high settings. Since the Guardian Games update, I constantly dip below 60 and hardly go above 70 except in menus. Changing settings barely matters. Many other people both here and on Reddit are complaining about it.
Revert whatever you did ASAP.
Same here. Stopped playing a couple weeks ago, came back from smooth 60fps to skittering between 40-60fps with obvious stutters. Tried tweaking settings, deleting and disabling GPU shaders, installing different drivers, checking game file integrity, deleting CVARS, fiddling with latency modes, etc. No issues in other games or in game/general benchmarking.
Rig: 2070 w/ 8th gen i5, 16G RAM. Destiny is installed on an SSD. DxDiag [url=https://pastebin.com/zPsjyaxs]here[/url].
Other users seem to be having similar issues, such as this Reddit [url=https://www.reddit.com/r/DestinyTechSupport/comments/mvik89/sudden_decrease_in_performance_one_day_no_idea/]thread[/url].
Edit: link wonky.