There are lots of possible reasons (although it is unlikely Apple is plotting against you
) Have you checked the iPad storage? Probably you have a lot less space than previously and even if it’s not full this will slow you down. Also a lot of imported media can be in an inefficient format. Maybe you are using more complex files.
Generally if your media is in the Photos app (on device not iCloud) it should work well in LumaFusion. Files imported to UserMedia from other devices might be slow the first time they appear but ok afterwards because LumaFusion makes a project copy. Using the optimise and consolidate option for a project can also speed things up.
I hope this gives you a few ideas.