I offer this story in hope that it may help someone else with the same problem.
I copied a large number of samples onto my Play SD card and then noticed that suddenly it was taking up to two minutes to load a project on the Play. I assumed it was related to my recent sample additions and deleted them. No change. I reformatted the SD card and restored it from a backup from before the sample additions. No change. (This was the original SD card that was included with the Play.)
Finally I acquired a brand new microSD card (64GB C10 U3 A2 XC1 V30 - whatever all that means), loaded it from the backup and added all the samples I had previously added, and more. No problems - I think it might load projects even quicker than before.
Use SanDisks Extreme ore Extreme Pro SD cards and never look back. No reason to go for cheap or random at-hand cards, if prices are what they are for storage.
Don’t know if this is your problem, though, but never ever had problems with high quality storage.
It could be related to amounts of patterns. I noticed when I have a lot of song parts in pattern view it takes a ton to save and load the project.
If you can, delete if there are any of the patterns you don’t use it might reduce the load time a lot