Ideas
I am also having this issue repeatedly. I have really noticed it recently as I am trying to make my own custom save with builds from the gallery or from my library. Usually, the first 1 or 2 lots I place goes fairly smoothly. But after a while, the game seems to take longer and longer to load into the lot I placed, or back to manage worlds.
I can't really see a specific pattern in terms of specific lots or worlds, as this has happened to me across different worlds and lots, as well as both residential & community lots. All the builds I am using have no CC. I do play with some mods/CC, but I have encountered this issue regardless of whether my mods are in my mods folder or not. I always clear cache when adding or removing mods & I very frequently check if any mods are broken and/or need updating. I have also had this issue on a clean install without mods on my laptop (I usually play on my PC).
I am not fully sure if this is a separate issue or related to how long the game has been open for in one play session (which I saw is in another bug thread). Potentially it is both, as I do find closing and re-opening my game can help. Sometimes, I have to force quit the game because it doesn't seem to finish loading. Sometimes it takes significant time but will eventually load - it can be in excess of 20-30 mins, at which point, I typically decide to force-quit and re-open the game.
This has primarily impacted me with loading into lots and back to manage worlds. I don't experience as much lag when playing in live mode, or CAS, at least most likely not of the same type.
I have all DLC up until Garden to Table kit (but not Spongebob or Prairie Dreams kits).