I’ve heard recently from a couple of people who found that accessing SharePoint folders via mapped drives in Windows Explorer had suddenly become very slow – even taking several minutes to open a folder. This is in Windows 7, but the same might (or might not) apply to other versions of Windows.
SharePoint folders in Windows Explorer use WebDAV (Web-based Distributed Authoring and Versioning) under the hood, so although it looks like just another shared drive it is actually using HTTP calls to list the files. It is useful if you are out and about, because you can get at documents on your internal network over the internet, using SSL to secure the connection.
The fix that has worked in both cases is a mysterious one. You open Internet Explorer (even if you use a different browser), go to Tools – Internet Options – Connections, click LAN settings, and uncheck Automatically detect settings.
I am not sure why this works but presumably with this option checked there is some sort of useless auto-detection going on which times out and then repeats.
No promises; but making this change can dramatically improve performance.