From what I've read, this is an issue that has been going on for a few years now and I can't find any fix for it whatsoever, so I thought I'd make my own thread about it. The first instances of this problem seem to occur around early 2013, so it may be to do with an expansion that was released around that time.

I recently went out and bought some Sims 3 expansions, and since installing them I've had this problem. I never had it before installing the extra expansions. I've also added some CC and store content to my game.

Here's what happens:

When I click on any Sim - including pets - in my family (to open up the interaction menu) I get a five second delay before the interaction menu pops up. During the delay, my Sims freeze and I can't click anything, but the PlumbBobs and zzZZs still move. It gets REALLY tedious when I'm trying to get my Sims to interact with each other. When clicking on the active Sim, the delay lasts even longer. I don't get any sort of lag besides this, and the interaction menu pops up immediately when I click anything that isn't a Sim... so I must assume the problem is something to do with scripts that are run when you click on a Sim. It's definitely not a computer or game performance issue (that's what was suggested by most people in other threads I read about this problem) because my specs are much higher than required & I don't experience any other sort of lag.

I've tried clearing my caches and removing all CC/mods/store content & the problem persists. My game is at patch level 1.63.

It's becoming game-breakingly irritating

Sigh

I have the exact same problem and it's driving me crazy because as you said, there's no lag when clicking on anything else but sims and it makes the game so tedious to play! I have tried numerous 'fixes' from numerous different forums and nothing seems to work. I'm at my wits end because all I want to do is play!

This type of UI lag and the game freezing when doing certain things (create a style) started around patch 1.55 maybe before it, I know it was around the time University released I think.
With Island Paradise I see dive lots are causing the game to lag/freeze when you enter build/buy mode and try to edit the ground in anyway, flatten/soften etc.