Can you please clarify if by "loading" you mean --
-- the app going to the server to completely fetch message text (fetched only partially during mail check)
or
-- the app's UI being sluggish, and the on-screen elements being slow to appear, like the message header, that sort of thing?
If the latter, please try Menu -> Compact the database in the app's main window (account list).
If the former, this is heavily affected by mail server and network performance, and these thing can vary, and are outside Aqua's control.
Feel free to capture the issue in a debug log, though (see below).