Okay, I selected a whole field full of sprites & models of plants that I had, and ended up copying them out and building them as one entity, which I then loaded back into my level, replacing the mass of single sprites and models that were there before.

Success!

The level compiles, no problems. In fact, it compiles even faster than before (as expected). In preview mode, the framerate doesn't appear to drop noticeably, except when I swing the view around on a line with the rest of my level, where there are many more single sprites. I think if I therefore combine as many close sprites as possible, then load them back in as entities, that will definitely keep my object count down. The only issue I've seen so far with this is that when using sprites alone (with no blocks), I don't see anything as far as placing the items, except unless I render the entitiy as a "box", then there's some trial and error in lining it up correctly. But that's fairly minor, as compared to having to build and place all those sprites again (they are on uneven ground, at uneven angles, and there are many of them, so I'd rather avoid re-orienting them all again).

I think my level has slowness because it is one really big building with lots of open space and windows. I don't know what I can do to get around that; the manual says build rooms with line-of-site in mind, use twisting hallways, etc, but I don't have that option because this is based on a real building, and people would not like it if I started changing the architecture on them.

Can I do anything else to get framerates down in my level? I've read a few things about clipping distance, but I'm not sure how that's going to look...