If you switched from using the SteamVR plugin to the OpenXR plugin, it changes up the frame synchronization a bit. Not sure if it has any effect on performance though.
The SteamVR runtime synchronizes on xrBeginFrame (even though it’s supposed to synchronize on xrWaitFrame according to the OpenXR spec). It’s blocking on it for longer than a single frame, which means that SteamVR is throttling the framerate of the application. It’s supposed to do that when it detects that the app isn’t meeting framerate, but will try to drop out of it periodically.
To make it easier to actually see the frame timing, you can disable the throttling from the SteamVR settings under Per-application video settings, by setting the Throttling Behavior to Fixed, and leaving the slider at the highest framerate. The SteamVR performance graph should turn orange instead of yellow if throttling is disabled.