...........but the Inspiron 620 is not in the list of tested PCs on the Dell website so they do not recommend it.
It's probably not worth the risk. Anecdotal reports show that some 620 users have had catastrophic problems. There is of course the option to revert back to your previous Windows version if Windows 10 doesn't work but that can cause even more problems with some users.
.........has become so slow as to be almost unusable.
Would to like to troubleshoot this and hopefully fix it? I have to say that it may take a bit of a while to fix so let me know in advance if you've got the time and perseverance. If you'd like to proceed with this then let me know your version of Windows.