Building outside sim borders

I've visited two sims recently where they have used tricks to push prims outside the borders of their sim to place things in the no man's water around the sim. In both, lag seemed to be pretty bad for me, and I wondered whether these prims might be responsible.

I remember being told a long time ago that it wasn't a good idea to allow prims to extend beyond the sim border... but does anyone know the technical reasons why it isn't a good idea... or am I wrong about that?

I am also curious about this, I have a few prims like that in a small build I did for an island sim. Never heard this and haven't noticed any particular problems in my case, but of course every little bit of sim-performance optimization helps.

As far as I know, the only downside to prims extending past a sim border is that if one is in the next sim over, the prim appears to extend into that sim but you cannot collide with it. So it is difficult to have buildings spanning a sim divide for example: you fall through the floor when walking between sims because of this sort of problem.

If however the prims are extending out into a non-sim square of the map, I'd be very surprised if there were a performance hit. That would have to be a very strange bug.

On a single sim island I haven't found any problems with this. You obviously can't put the centre of your object outside the sim, but if part of an object extends past the sim boundaries there is no problem that I know of. I've seen a few people put waves that start outside the sim.

a perfect example of this is at the Ben and Jerry sim.. they are using the largest prim in World.. for thier water. to make it a solid water cartoon look.. if you look its a single prim and it extends way past the horizon. as far as i see it dosent lag out thier sim at all.

There's no side effect associated with prims extending outside of a sim borders (in islands) except for not been a useful structure other than aesthetics, since you can't walk or fly beyond the border. The prim(s) center though, must be within the borders.

Hummmm. I'll have to see if this would allow me to position my jumping dolphin a bit further from shore... If I stick it on the end of a long transparent prim, and put another prim at the shore end as the root prim... Make the whole thing phantom, since anything past the sim edge will be phantom anyway...