I have not been playing very long, and am only using a single sim family, but, so far, lag free. Wish I could say bug free too, but, that's another issue which seems to be affecting just a few players and which I have found a work arund for until I can get a proper fix, or it clears itself.
I have found in other games, that lag occures in the larger maps, such as IP more often than the small maps. I downgrade my graphics if it becomes annoying and this generally fixes the issue. If it is pathing, then resetsim * will give a temporary fix. Try this, and if it fixes the problem in the short term then you will need to find the locations with pathing problems.
I'm sure, if you find them, others here will be delighted to know. There are a couple of venues in WA which cause major lag, but, these are easily fixed by changing them to no visitors allowed before playing the map. Your sim can still go there, but, NPC's will not. So, this may not be a suitable fix for IP.