There is an option to stop Unity from automatically checking if files have changed and and reimporting them. Might be useful if you don't need to see any update of your work in Unity, like when you just add your assets without wanting to work with them right away. But this has the big drawback that you will have to manually refresh it. Not too good, but you get to decide yourself when the reimporting and all runs and schedule a meeting to fill in. ;)

It would be great if one could mark folders with "don't automatically refresh" flags or something.

We also feel this is an issue that needs to be addressed.

You should certainly contact support@unity3d.com or else file a bug report for something like this.

Try disabling "Compress textures when importing" in the Unity Preferences (Edit/Preferences).

If that doesn't work maybe you could try contacting the Unity team directly, as 15-20 minutes is a first for me.

This only works in very specific situations, but if you have assets that you load procedurally (for example, you load a file and use Texture2D.LoadImage to convert to a texture), then those assets probably don't need to be processed by the asset importer.

You can prevent them from importing normally by changing their file extension -- my project has 100's of ".jpg.x" files which are loaded procedurally as individual frames of texture animations. The phony extension means they get imported very quickly with the default (do-nothing) importer.