The first thing I would want to know is how long it takes to start up a game in the same world that is new or has not progressed so that there is a basis for comparison. If it takes 11 minutes to load up a newer game, then we know the issue is not with this save. If it takes 2 minutes, then we know that it is and it's not your package files or other forms of content, etc. as those would be common to any game save on this game folder (although the time for comparison is more likely to be someplace in between those extremes).
Edit: I misread the OP and got the loading time backwards, reading it as 21 minutes. A loading time of 12 minutes for a progressed world with that much stuff on the lot is not unreasonable unless you are running the game on an SSD which should be cutting that down to a fraction of the time.
I can't prove this exactly, but it's always been my sense that hundreds of items in storage containers like chests do not have the same impact as hundreds of items hanging out loose in inventories or on the lot so that may be worth pursuing if they aren't already stowed away like that. The 200 photos on the walls of the actively played residential lot are possibly having a huge impact though.