So sprite importer isn't intended to stay for the duration of your project, it is intended for project startup only. Once you start working with the files you should remove it. The problem you're experiencing in your other post is that sprite importer is reimporting all those sprites everytime you mess with them. So before you mess with them, you're supposed to install sprite importer, then setup your folders how you want them. Like I have a spritesheets folder and every sprite in that folder is sliced 16x16, where my characters folder is sliced for whatever the dimensions of the characters are. I have separate sprite importer settings for each folder. Then when you've finished adding all your sprites, you remove the sprite importer to avoid having the other issue you're having.
The reason for this performance issue is due to a failure on the part of Unity's designers. It the cost of having automatic sprite configuration on import. There is a faster method but it requires that you slice them individually one by one. It's almost as involved as just doing it manually yourself the way Unity intended.
Also I am curious why it took an hour to slice all your sprites. Are you doing this on a PC from 1995 lol. It takes my computer 16 seconds to slice every sprite file from Seliel the Shaper's entire collection.