Lag Due to Other Lots in Neighbourhood

@BansheeHVO  If you want to test whether the issue is the lot itself or the save, try placing that lot and your current household's lot in a new save (but in the same locations), then play as you're playing now and see what happens.  It's always possible that the issue is a sim living in the other household, or even some object in their inventory, odd as that would be.

I don't know enough about how the game engine handles all its calculations when dealing with objects that are on an inactive lot that is currently in camera range.  I do know there's a difference—if you want to see it for yourself, go to a central location in a large-enough world, for example Sulani, and watch framerates change as you move the camera around and zoom in and out.  That even happens in a new save with nothing added to it.

This is what I mean about the game engine having limitations: I don't know exactly what's triggering the fps drops, but I know it happens before I build at all or my sim acquires any inventory objects, with no mods or custom content installed.  And I know it's not my computer's hardware because I can see from monitoring tools that neither the CPU nor the GPU is anywhere near maxed out.

Anyway, this is the kind of problem you probably can't solve, just work around to some degree.  As for why it started happening a few weeks ago, maybe it was the last patch before this one, or you added something to the large lot, or a sim who lives there acquired something or changed in some way, or the eco footprint changed, or the season changed, or... it can be really difficult to tell if you're not paying close attention at the time.