FastMail likes to encourage people to find the best way to access their email. This might mean that at home you setup an email software like Outlook Express to access your email via IMAP, while at work on on the road, you might use the web interface.
One issue with this has been that FastMail keeps a list of folders in the web interface that's separate to the IMAP server. And while it would synchronise the web listing with the IMAP server when you go to the Options -> Folders screen, it still confused people regularly when they'd create a folder with their IMAP client, and then login to the web interface and couldn't see the folder!
Part of the reason that we didn't sychronise the web listing automatically on login was that listing the folders on the IMAP server was a relatively expensive command that could take some time to complete. However recently we came up with some optimisations that made the listing of folders quicker. So we added some code to synchronise the folder list on login, and also to display a short welcome message between logging in and transferring to the mailbox screen.
Despite the optimised sychronisation time, the slight increase in login time has annoyed some users, especially those that don't use IMAP clients and thus have no need to synchronise the folder list regularly. Because of that we've added an option on the Options -> Account Preferences screen, a "Sync folders on login" checkbox. If you uncheck that checkbox, then it disables the folder synchronisation and removes the login redirect message. This returns the login process to exactly as it was previously. You can still manually synchronise the folder list by going to Options -> Folders screen.