Not sure if this is relevent to your problem but I had a similar problem with the frame rate dropping a while back. I would be collecting guns in my game (which were just long thin Blitz cubes used as placeholders, so no huge polycount involved) positioning them near a second camera used to render the heads up display graphics, and then EntityAlphaing them to zero to stop them from being rendered while they were not being used.
After collecting about 6-8 guns the frame rate would abrubtly drop from around 80 to around 40, even though the guns were not being rendered. I tried positioning the guns so they were not all occupying the same space in case it was some sort of weird z-buffering issue but it didn't seem to make a difference.
I eventually fixed the problem by using HideEntity to hide the guns instead of EntityAlpha, so if you're using EntityAlpha in your game it's possible it may be causing the problem you described.
After collecting about 6-8 guns the frame rate would abrubtly drop from around 80 to around 40, even though the guns were not being rendered. I tried positioning the guns so they were not all occupying the same space in case it was some sort of weird z-buffering issue but it didn't seem to make a difference.
I eventually fixed the problem by using HideEntity to hide the guns instead of EntityAlpha, so if you're using EntityAlpha in your game it's possible it may be causing the problem you described.