Since there aren't widespread reports of similar behavior, it would lead me to believe you simply had a bum installation. It is possible that by uninstalling 7.3 and reinstalling a previous version, you cured the bad install.
When I see a user having a difficult time with a fresh install, one of the first stops is to uninstall and reinstall from a fresh download. That usually clears it up. I would recommend your giving Lightroom Classic CC 7.3 another go and, if you still see the same problems, post some system information, steps to create the issue and some data about the issue itself (i.e. how many seconds in 'forever' from your original post.