I've barely played the sims 3 since April, maybe even before. I didn't even have that many performance issues, slight lag maybe. I just got bored of it. It's partly my fault as I tend to skill, skill, skill. With the sims 4 though it's new, fun and the multi-tasking is brilliant. I'm trying to focus more on the gameplay now than just getting to the top level of everything.
The sims 4 really is more focused on the sims themselves, I don't think 3 had that.
Yes there are things I prefer in 3 but to me it had run it's course long before 4 came out. Saying that, I spent a lot of money on 3 so I can't see me uninstalling it just yet.