The app has settings for how much of each message it should load when checking mail.
The defaults are very conservative (for slow devices / mobile connections), so you may want to increase them.
These are global app settings, which show in Extended mode (top item), and are almost at the end of the settings list. There are separate settings for WiFi and mobile data.
In addition, IMAP accounts can be configured to preload embedded images. This is under Options and Folders (long press the account in the message list, choose from popup menu).
