I imagine this will be extreme nightmare to implement, but I'll make the suggestion anyway.

Assets used in the save game (or map itself when starting a new game) would be fully loaded as usual. All other assets that need loading would only be partially loaded.

By partially loaded, I mean just enough information to let the game know the asset exists, so that it can be shown in build menus, spawned, etc.

When the game actually comes to place the asset on the map, however, it would need to complete the loading of that asset. This would cause lag and stuttering - something I'm completely OK with if it means a significantly faster load time and much better in-game performance in general.

I assume the mod already caches the "shared resources" information of assets (if not, why not?) so assuming the asset has already been fully processed at some time in the past (and not changed since) it would be possible to work out which resources need loading and which could reference an already-loaded asset? Whenever a new or updated asset is detected, the mod would fully load it (assuming it makes sense based on mod settings) so that it can check for errors and cache metadata about the asset.

This really would be my dream scenario for new cities - I want a very fast load time, and very fast in-game performance. And I really don't mind the game suddenly stuttering for even up to 30 seconds whenever it tries to spawn a partially loaded asset and realises it needs to finish loading it. The improvement to overall performance the rest of the time would more than make up for the annoyance of game lagging each time an additional partly-loaded asset is encountered.

There would likely need to be some exceptions - eg. assets in mods, road prefabs and so on. Trial and error will determine the specifics.