To followup on your response: Your settings seem to be reasonable, nothing that would warrant a slow response (from Aquamail's side).
As Kostya pointed out that you can enable preloading embedded images, - and that's
6(b) setting that I was asking you about:
6. What are you settings for: Long press on the account name (in the account list view) -> Options and Folders -> Preload ->
a) Attachements, WiFi
b) Embedded images, WiFi
6) 500/500
Rather than this, - the slowness might be caused by too many other apps running and using up RAM (so that Android has to free up some RAM first, before loading your message).
To test this, you can try looking at the same message after rebooting the phone.
(There could be still too many apps running on startup -- automatically.)
Yet another reason could be if you have very little free space on your internal storage.