I just finished a good two hour map session in the SDK, everything seemed to be fine. Try restarting steam, your computer, defrag, checking integrity of game files, etc etc?
If all else fails, shutdown Steam and delete the "ClientRegistry.blob" in your "C:\Program Files\Valve\Steam" folder. Then reboot Steam and you should see an "Updating Steam" message as it re-downloads the "ClientRegistry.blob". This fixed a similar problem I had.
If this is your first time logging in after the migration (Feb 8, 2022), you must reset your password to log in. Follow this guide if you're having trouble