If you have many local versions saved on your computer (as shown in the attached image) the launcher takes a very long time to load. I guess it's because it checks every version to see if it's the same as remote.
One way to recreate this issue is to download many versions to your computer and then restart the launcher
Ways to fix this:
1) Only check a version once (like after it's been downloaded).
2) Only check the selected profiles version each launch (this makes it so that the version you play always is checked).