not a Mac person, but broadband speed should not affect the operation of LR/Classic except in rare situations where cloud syncing of files outside of LR is in play and even then it is not common. However, malware can slow things down behind your back as some such programs commandeer your computer to do things not related to what you're doing - for example sending spam emails to 10 million stolen email addresses using your computer to do the dirty work.
A 9 or 10 year old computer is pretty old in today's world and unless it was a top performance model back then may be bogging things down but that would not be related to the newer version of LR. . Chip speed could be a factor but in most cases more memory is better at speeding things up.
You can try reinstalling the latest version again. but, you can also drop back to your prior version as long as you still have the LR catalog from that version and don't care about losing any work you did since upgrading (there is no "downgrade catalog" tool).