Site Slow after Turning off Multisite

Hi our website is really slow and getting slower since we turned off the multisite. I’ve noticed that when we use a plugin to create a backup such as All In One Migration it takes forever to take a copy of everything which is a big clue. It shouldn’t take so long. Its just a shop with about 300 products. There was a multisite set up for a small additional shop for b2b but that only had about 10 products. It has been removed but I think some files/tables still exist and are making everything too slow. How can I find out what is causing the problem please? We are trying to add some more products and its taking forever.