I’ve had the same lagging issues!
1) when my sims are changing levels or using stairs at any time the camera must be on the same level or they’ll just stand at the top or the bottom of the stairs. This can get really frustrating with larger households, as no one can autonomously do anything without me constantly checking that they made it up/down the stairs. It took me forever to figure out why it took so long for my sims to do things, and then I realized they were standing by the stairs all day!
2) I noticed some lag even before the update, my solve currently has been to open and close the game menu, that normally “resets” the lag when switching between build/buy and live mode.
Yes - I had some lagging issues before the Cottage Living Pack BUT - it was not as bad as it is now - afterwards. Before - My lags were usually with the Sims doing action (so as you said - using the stairs - going to bed - using the toilet) They would just freeze... Not do anything - put their hands up in the air and shake their heads... I found I could almost fix it - make the game playable - if I paused for a minute - let the computer / game think and catch-up with lag issue. NOW that Cottage Living is installed though... LAG EVERYTHING!!! Lag for the Sims to do any Action, Lag between switching Sims, Lag when I try to put an object into inventory, lag when I try to do things like add traits or select an action on the chart... LAG when I am buying items in the buy MOD. It is simply so laggy - I cannot play it this way - No fun. I am not sure if it is just using up too much memory now? This Game Pack may use SO MUCH memory - it is making everything lag... I have even had the game crash a few times (haven't had that problem with my new laptop on the Sims 4 at all since before Cottage Living.) I do use custom content and a lot of it... BUT when I removed the CC - It is still just as super slow... Is there a fix? Is EA or Maxis going to give us some help or tips? Are they even going to Acknowledge this is problem many of their users are having? All I know is I don't buy new things (packs, etc.) On a game that I cannot really even play... So unless this gets fixed - I won't buy anything further Sims 4 related (and I own EVERYTHING they have released up to this point...)
