Loading mails is not exactly fast.
I guess one part is the WebView not being all that fast, so we have a slight delay before we can see the mail.
A quick check shows that the OTP is done generating HTML typically in ~5ms tops for regular mail (fancy graphical mails maybe ~10ms) , but as soon as we have a signature it takes ~500ms.
An encrypted mail takes even up to ~800ms.
Perhaps we can improve this to some degree, but otherwise we would probably have to improve it by processing the mails ahead of time. The primary usecase for fast switching is if you go through your list of mails I think, so we could i.e. always prepare a couple WebViews for the next mails and thus make switching feel much faster than we can realistically achieve otherwise.