I have laggfree no freeze Islan paradise now, and I have not changed too much like others do.
I did have major lagg, totally unplayable. I did not believe it was possible to fix but after raging and reading forums, editing and testing I made it.
This is what I did.
I downloaded the old fixed map:
http://modthesims.info/d/514909(This map in itself does not solve much lagg problems, you have to do a lot youself afterwards)
Then I evicted families on houseboats and moved them into other houses on the island, then I removed ALL houseboats. This helped a bit, I'm not sure if its necessary to remove all of them but I did. (Dont delete families, some of them are important for gameplay so keep them in other houses on land)
I moved collins famliy, their house is cursed and cause a ton of laggs.
I removed medina house totally and another one, I think it was amigo brothers or something, 2 guys in a house on the shore on an island. Sims get stuck there under their house.
The castle, I removed all stacked spiral stairs and made ladders instead, and also fixed some minor things to make it easier for sims to move around in the house.
You could if you want just delete access to any room that is not necessary, I did that for one house and that fixed a routing problem, it was one of that small houses close to townsquare, top floor is empty and just remove ladder/stairs to it.
I went to at least half of all houses with people in it and removed at least 1 arch if it was infront of the front door and removed the wall the arch was on.
And I made sure "front door" was properly set on all houses, some houses have misplaced or no front door.
On 3 babes house, remove 1 pice of the long fence next to the wall of house, and make their porch a little bigger so they can greet sims on it.
I also made minor changes to houses that I don't remember, if you ever get the message about an unroutable sim, click on the sim picture in message to see where it happened and then edit the house/family to fix the problem.
After I did all this it was barley playable, it was smoother but still laggy.
Then I discovered the biggest problem ever that cause some INSANE lagg, the major source of clusterF pathfinding.
PETS, those PETS will break the game big time.
1 singel pet in an household with more then 1 person WILL very likely lagg the whole game!
I removed ALL pets in all families with more then 1 person, just merge a random sim into the famliy and then split it with that sim and the pets.
Now the game runs silky smooth, no lag at all!
If you have lagg, just remove all pets and fix front doors and such.
I have read in other threads about people fixing ports and such stuff, I did not do any of that. Ports and all commercial buildings are unchanged.
I also installed the mods, Error-trap, Overwatch.
The only problem I have now is there seems to be something wrong with vehicles spawning.
One night I had 76000 vehicles removed in nightly cleanup, yes 76000! Usually its 40-1000.
I don't know the source of this and I have not really been looking around for it, as it does not lagg the game of affect my gameplay at all. My guess is that some sim is stuck somewhere spawning all this vehicles.
I run the game on absolute highest graphics settings, 4 sims, 2 pets and 1 buttler in my household. No original families deleted, imigration and such is on, zombies and aliens are not removed like some recommend. It's not needed.
If lagg starts again after a few weeks silky smooth gameplay it's because some neighbor decided to buy a PET, then go to exit town and remove it and your lagg free again.
I did all this changes in 1 day.
From knowing nothing to fixing game, mabie 5 hours of work. As you don't have to read everything like me you can do this much faster.
There are some good threads that list houses with problems, I don't have link now but I will post it if I find it again.
It's a shame EA released the game like this, noway could this have passed any kind of quality testing.
Now years later it's still the same without any fix, they could easily fix this in just a few days work but the multimillion$$$ company EA did not even think about fixing this.. remember this. I will not buy Sims 4 because of this, they just take your money and then don't fix the broken unplayable game they released.
When you do this fixes by yourself, Remember!
Start from a new save.
Do ALL the necessary changes, then save your game before you make your household.
Then you have a fresh fixed save that you can load when you want to start a new fresh game with a new family.
TLDR;
PETS is the biggest source of the major freezes and routing problems. Remove them from neighbors.
Fix front doors.
Move a few families from problem homes to other houses.
Now silky smooth gameplay.