same issue here. I have an SSD Samsung 850 EVO. Around 20 mods and 500 assets installed. Needs ages to start a saved game or a new empty map, after new patch it needs even more time.
It's probably because of quite a few assets they are not correctly created. Output_log.txt is full of issues when loading assets. mesh out of bounds, many LOD issues, but I don't see a system. I can unsubscribe to a lot of assets but only that after unsubscribing other assets are listed with issues.
For me the whole workshop / asset stuff is a complete mess. If the assets have issues it should be denied to load them in C:SL to avoid problems. Obviously the game engine can detect issues with the assets (because the issues are listed in the output_log.txt), so when starting the game these assets with issues should be denied to load and a windows should pop up to disable or unsubscribe to these assets.
The stock Steam Workshop definitely isn't cutting it for this game. The big problem is with the growable assets, because you end up adding hundreds of them for it to make any noticeable difference in your city. Then you spot an asset with phosphorus colors, broken LOD, way too many polys, or one that simply doesn't fit in your city. Now you're left trying to find that asset in the workshop so you can unsubscribe - not an easy task when it might be called "Test" or "House 01"...
I think this game is eventually going to need its own mod manager, with better filtering/sorting capabilities and the ability to batch subscribe/unsubscribe as well as a better rating/curation system.
I used to have about 800 assets. I've disabled them using the Improved Assets Panel, but I still need to find te time to click unsubscribe 800 times in the Workshop. My loading time with 800 assets was only just over 3 minutes, but the LOD issues and other problems mentioned above became too much for me.