When I first installed IP I was having a lot of general glitches not just lag, I decided to do a reinstall as it had been some time since the last one, I also altered my settings after reading some people say that it played better on higher settings so I maxed everything then later tweaked it slightly to bring down maximum lots loaded to 4 and trees and visual effects to medium.
It played like a dream for the first few weeks, took me about ten weeks to finish the Grand Explorer lifetime wish and up until I took over Hobart's it was great, as soon as I got that resort my game would not just lag it would lock up entirely forcing me to use task manager to close the game, after I bought it I sold almost everything then went into edit town and bulldozed the lot to empty it completely, I regularly checked for spawners in case it was those but it wasn't, in the end I found the only way I could get that last island was to just leave it to run itself after I'd done most of the building and never to look at it unpaused as if I did it would lock up again
As soon as I got the island I sold the resort, and had no more problems, I also upped my graphic settings in my dynasty file when I went back to it and that is playing smoother too, I guess it's natural to think lowering them would help especially as the tooltips say higher settings might affect rendering performance but ironically for me raising them has made it better
I had pets turned off in IP and the fact the islands have no strays is a good thing I feel, one issue I found with stuck sims was showtime performers, they didn't pick good spots to perform for tips half the time causing routing errors with people trying to watch them, I wish we could turn those off too or that they only spawned if you added a venue.