When working on my avatar with complex constraints, some actions, such as Undo, will cause the editor to hang for >30s before resolving the action. This makes it very difficult to work with the editor.
Not every Undo will cause the editor to freeze, only those that modify the avatar in some way- undoing other things in the scene (such as moving a random cube) will not cause lag.
I have also rarely experienced some other common actions freezing the editor while working in the beta, but cannot currently recall which they were or how to replicate them.
If it helps, the avatar I am working on is an avatar with a 2nd armature of ~20 bones, each of which is constrained to mirror the main armature; the 2nd armature is then placed on a modified "amogus follower" to follow behind the main armature. I am using primarily rotation constraints, with only 2-3 position, parent, and lookAt constraints respectively.