Selecting objects in the hierarchy it takes 1-3 seconds for the inspector and gizmo to update accordingly.
I'm having trouble narrowing this down for a clean repro, however I have found the following:
  1. Only slow when selecting the avatar, or objects that are children of the avatar.
  2. The lag does not take longer or shorter based on what object you select, even an empty gameobject with no components (as long as it is a child of the avatar).
  3. Requires an avatar descriptor with custom animators set (resetting playable layers section or removing the avatar descriptor cures the lag).
  4. Requires vrc constraints somewhere in the hierarchy of the avatar (deleting all constraints cures the lag).
  5. Removing no one particular animator seems to make a difference, the performance almost seems to be worse based on the number of custom animators specified.
Also I should note that there aren't any complex or cyclical constraints setups on the avatar I'm debugging this with, just a bunch of simple aim constraints used for twist bones.