Honestly took me way to long to actually download older version and test, and just wanted to confirm with other users.
Viewport performance in 2.91.2 is significanltly slower then 2.83LTS…
In viewport, set FPS to 100, and used playback funciton to show FPS while rotating manual in the viewport around my model. View in just shading, no textures/materials.
2.91.2 - 8.3-8.6fps
2.91.0 - 8.3-8.6fps (same)
2.90.1 - 38-40fps
2.83.12 LTS - 41-43fps
AMD 6900xt, Windows 10
Going to test other versions as well. (NOTE these are clean installs of blender (deleted roaming files), no changes outside of the FPS option )