After clicking “Reply” to an email that has been fully loaded in the Preview pane, a new window opens containing the email so as to be able to write a reply. It can take up to 30 seconds to load the body of the email before a reply can be written.
I have two mail accounts one Exchange one IMAP and it happens with both. It does not appear related to the body content and whether it contains objects such as images, it can be a few lines of plain text.
It is not clear what it is doing in the meantime. Is there some setting which can be changed to stop it doing whatever it is doing so as to be able to reply without having to wait?