Oldspook, here are the things that I did that helped with the lagging the most for me in the town of Isla Paradiso. They make IP playable for stretches, but eventually that little stuttering gets under my nerves. Lol.
1. I keep only a half dozen to a dozen things in my sims backpack inventories. If I start getting too much stuff in them like I used to be able to do, the game really bogs down. Don't understand how this would affect things, but it sure does in my game.
2. When not actively using it, turn the collection helper (if you have one) to the "none" setting.
3. If you can stand not discovering them all on your own through game play, using the cheat to unlock all the hidden islands really improved lag while traveling by boat for me.
4. Overwatch mod from Twallan. It resets stuck sims and cleans up a lot of other "file junk" that tends to accumulate. But I have had to have it running since the Late Night EP.
5. Register mod, also from Twallan. I use it to completely turn off all the stray animals, wild animals etc. This world seems to have a lot of pathing bugs, and places for sims to get stuck. Being able to turn off 20 or so animal sims that always kinda bugged me in the game anyway eases the number that can potentially get stuck.
6. In edit town, I kicked all the townies out of their houseboats, moved them into other empty houses, and then got rid of the houseboats as well except for those that my active household owns. This one cut down on a lot of the lag/stuttering.
7. I cleaned up a lot of the lots that were built with bad pathing practices. Like a section in the Spanish styled resort's pool. There is a fenced in area in the middle of the pool, but still had something inside that fence that NPCs like to "view". The game in this situation, has probably thousands of potential tries to get to the object inside with routes over the pool, but the fence negates the path each time. Yet the game's pathing engine is going to run through all the options every time an NPC thinks he needs to check out that object. Another situation in houses, were circular stairs with walls placed in ways that often make them unusable. I replaced these with ladders most times or moved the blocking walls/objects. The castle looking house, where the Ichtaca family lives, had a number of situations. Once I cleaned them up, Overwatch stopped telling me they were resetting one of the family members ten times a day or so. Another area was objects in closed in areas that were only accessible by the garage door style of doors. These don't auto-open for sims, and for some reason NPCs can't open them in their pathing either, so again the game potentially has to try to figure out all the potential options before it gives up. I noticed this mostly in the modern resort lot. The VIP rooms have little outdoor deck areas that are not accessible any other way.
8. I got rid of the Scott family. Several people on the TS3 forums have said they were saved in the town in the middle of an activity and that object isn't there anymore. Or something like that. Those with the Error Trap mod, say it goes nuts over this family. So they had to go from my game.
9. Don't go crazy with resorts you may build. Make sure any island resort you build has ample empty coastline for sims to enter the lot. My first resort had tons of decks and jetties over the water to the edge of the lot. My game went nuts with the lagging until I just bulldozed that resort. Keep pathing in any resorts you build simple with redundant ways to get to just about everywhere.
10. As counter-intuitive as it sounds, turning my graphics settings all the way up helped significantly.
These ten things above cut my Isla Paradiso lagging back to about 5-10% of what it was when I first started playing it. The amount that it still lags is definitely playable and shouldn't bother me, but it does. Lol. So that is why I came up with step eleven.
11. Moved my sims I am playing back to Sunset Valley for the duration. Then found out how to get dive lots to sorta work there too.
For a few people on the EA forums and others that I read, one of the above steps completely cured their lag, so they might be worth checking out. Isla Paradiso really is a beautiful EA world.