Hello,
Do you mean the app taking a long time to sync mail with the server, or that the app's UI in general feels sluggish?
If the former, please check the "messages to sync" setting. This is in app settings under Mail, receiving. The default is 25, a much larger number (e.g. 250 and above) can be slower.
If the latter, please try Menu -> Compact the database in the app's main window. On some devices, the app's database gets slower over time and needs to be "optimized" to make it fast again.