This isn’t a common problem, so it would be good to mention any unusual aspects of your configuration. Where are your data files (on a network drive, the system “drive”, USB drive, etc.)?
If you are using Windows, you should mention the Windows version whether you use “OneDrive”, and which AV software you have. Some AV products move “suspect” files to quarantine or block suspect applications, so you should check your AV’s logs.
On Windows 10 (Pro and Enterprise) there have been problems (perhaps related to a recent update) with app windows taking as long as 20 minutes to load, so you should try extreme patience and watch for “Not responding” messages. I had this problem but it seems to have ended with an unscheduled update a few days ago. The problem seems to be related to a “recent files” list where some files were on a server that was off-line.
For linux, please mention your distribution and version.
For macOS please mention your version.
For both linux and macOS: If you have tried installing, updating, or running snap using the “sudo” command (e.g., with “root” privileges) you may have file permission problems.
If you can create a new “snaptest” user and do a fresh install of SNAP as that user it could help pin down the problem.