I have performance issues with both games, but for very different reasons. In PC, the guests are not the issue, I can have thousands of guests and as many rides as I want, the problem is scenery. as soon as I decorate just a few rides/shops I get a huge FPS slowdown. I loaded a large park running at 7FPS and closed it and removed all the guests it only made a couple of FPS difference. I reloaded and removed rides instead, the same. I did the same with paths, bins, minimal decorated shops .. only when I reloaded and removed scenery did I see a huge difference, I was adding FPS for every decoration I removed. I now play the game with minimal scenery.
In PZ its most definitely the guests, every time the camera pans past a large group of guests my game drops down to 7 FPS, which is basically unplayable. This doesn't happen in PC. I cant view anywhere near my entrance, or anywhere near a group gathering. These group gatherings are nothing like the PC gatherings - they are small compared to the mass crowds that flood a whole area in PC with little FPS impact. just a mildly busy path can slug the game down to 7 FPS, which last night resulted in a crash. Minimal scenery also has a much worse impact than in PC. I noticed a 2 FPS difference for each frontier made planet zoo theme shop I swapped out for a classic theme shop.
UPDATE: I have just reloaded my crashed game from last night. it was running at 7 fps for a couple of hours before it dropped below and crashed, its reloaded to just a few minutes before the crash and its now 23 FPS.