I have a similar problem since some time now. The game shows severe lag spikes at times, most prominently while using the build mode. Whenever I enter the build mode there are spikes of very awful lags. I discovered two things that help against those lag spikes:
a) save the game while in build mode. The lags are gone for me after that.
b) hit the Esc-Key on the keyboard (once to open the sub-menu and a second time to close it again). This - absurdly enough - gets rid of the lag spikes in build mode for me.
If you haven't already I'd recommend to try those solutions. I can't remember though if they work in live mode as well - the spikes aren't as bad in live mode for me.
Between the first time I realized the lag spikes and the day I tried the Esc-key solution I factory-reset the game twice, made a full clean reinstall once (during which I removed every sims-related folder and file on my computer that I could find), updated my windows and my graphics card and run the game with a new save and without any mods, all because of another bug I had (the now fixed broken social profile bug) . None of that fixed the lag spikes though. I still have them. I run the game on an AMD Card (for many years now), on a desktop computer.
Of course those solutions are just workarounds. They don't solve the reason why the spikes are happening in the first place.