What causes physics lag when there are no physical objects?

One of my tenants has a pretty popular build that generally has between 6 and 20 people in it at any given time. When the place is populated though, the sim suffers sporadic but severe lag spikes. I've been there a few times when it happened and the sim stats always showed the Physics Time stat bumped up to the low 3-digits (as high as 300ms), and this basically brings the sim to a halt when it happens.

Thing is, there are zero physical objects in the sim. None. I know that avatars are physical objects though, so I assume it's something going on in the build. But what? What could avatars be doing that would cause a sim to freak out like that? I need to diagnose this problem or I'm going to have to shut this guy down- and I know he's not intentionally doing anything wrong.

Well, if he can get the avatars to sit on a prim, they'll give the physics engine a break.

Also, if there's a non-phantom megaprim around, Havoc may be working very hard, trying to figure out if an avatar has bumped into it. After you get a few avatars clomping around a few non-phantom megaprims, things can get pretty laggy. (Oh, besides non-phantom: if megaprims have llVolumeDetect set in a script, it would have the same effect.)

It's also possible for certain attachments (e.g., flight assists gone bad) to dramatically amplify the amount of physics processing an individual avatar consumes--but there's not a lot you can do about that.

All that said, though, I've been seeing huge physics spikes on some sims that don't really seem to be related to anything at all, and appear to clear up for a while after a sim restart. I've sorta decided to just ignore the problem till Havoc 4, when we expect much better physics performance.

The question is: When is the physics update coming? Windlight still hasn't hit the grid and there was a preview version of it months ago. Thanks for the tips, though. I will pass this info to the build owner and see if he can identify any of those issues.