Weird performance issue

I'm having random slowdowns when playing a scene in the editor so I launched the profiler which gave me the attached information for Physics... So apparently I have

2.5k active dynamics,
2.5k active kinematic objects
12 static colliders,
2.5k rigidbodies,
2.0k trigger overlaps,

and the killer:

3.4M constraints

That would explain poor performance if it was true, but this particular scene has no rigidbodies, no colliders, no physics of any sort going on - it's just UI...

Does anyone have any idea what this might be?