Do you mean opening a folder's message list?
Try Menu -> Compact the database in the app's main window (account list)
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Or message text, when you open a particular message, slow to show (10-30 seconds, or even longer)?
Check system settings -> accessibility.
If you find any apps listed there, especially password management (LastPass, etc) o "security" related (Norton Something Or Other) -- turn them off, reboot the phone and see if the issue goes away.