Unity very slow with big project(s) - how to optimize ?

Hello,

I'm currently working on a project that has a lot of files (my context is : very graphic game, lots of small/medium/big pngs files).
The more month passes the more time it takes to open the project in the morning and start (play mode) a session. Somehow after the first time "playing" (doesn't matter what scene is used (heavy or small ones)).

My guess is that the editor needs to refresh temp files and load stuff into memory. The more files there is the more it needs to process to be sure to show them / be able to interact with them :shrug: ?

Any pointers on how I would be able to debug what's taking time to load ? I know about Profiling in editor. It doesn't quite help for loading and first launch.
Best practices on what to do and don't do when working with a lot of files so that Unity is still useable ?

Thanks for the help !