Mail app extremely slow to download messages

I've checked my gmail account through Mac Mail for years and had no problems. I've used both POP access and currently am using IMAP. Things have worked well for quite some time. Recently (within a week or two) message downloads via IMAP have been VERY slow. Mail shows messages only being downloaded at 2 or 3 Kb. Attachments take even longer to download -- sometimes up to an hour. Anyone have any idea why this started happening all of a sudden and what I should do to fix it?