If you read the bug report rmerren linked to, it says how to do it, but there is a lot of extra info there.

First you need to have root access, I'll assume you already know how to do that.

Edit the file:
/var/lib/gconf/apps/modest/accounts/your_account/%gconf.xml

(where your_account is the actual name of your account, use tab completion if you don't know it)

In this file will be a bunch of lines like this:
<li type="int" value="160"/>

(along with other XML stuff)

You can delete those "li type" lines like i wrote above. Save the file and repeat for other accounts you have. Then reboot your phone. Hopefully then mail will be fast after that.