I7 3570k
16gig of ddr3
SSD
GTX 970
With 30 mods it taks 30 seconds.
so, if we make a projection... 500 assets = 500 sec ?

Btw, are you talking about mods or modified / newly created assets ? I suppose the mods only don't need so much time to load. The assets can easily sum up to one GB or more, so I assume they are the culprit for long loading times. Otherwise, an asset that is not used in a save game, does it get loaded into memory ? Questions about questions we don't know to answer....
Also a saved empty map needs long time to load, so probably the subscribed assets contribute to long loading times.
It would be really helpful to get a guideline from CO what we have to observe when subscribing to assets (and mods).
How many assets to subscribe is recommendable ?
Do we have to care for file size of the assets (single assets and total sum of all assets) ?
Are they safe to subscribe if no error occurs (as e.g. "broken asset" or anything similar) when starting a game ?
Do we have to check always the output_log.txt for errors after subscribing to new assets ?
If the assets don't meet the requirements (mesh out of bounds, Unsupported format [Importers, Native - Internal], very high vertices count, LOD errors etc etc..), it would be good to just strictly refuse the load of these assets (and a recommendation to unsubscribe) by the game itself - if these messages mean the affected asset could crash the game or lead to malfunction.
And pls don't give control of the output_log.txt to the users - without knowing the meaning of all these messages in the output_log.txt this is useless. We don't know what is a severe error and what is only an information to ignore. Besides this it's very time consuming always to check the output_log.txt for possible errors.
At least there should be simply a popup window "asset xyz doesn't meet the requirements and won't be loaded, pls unsubscribe" - this would be already really helpful and would avoid problems with the game because of badly designed assets.