We have 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.
TO FIX SIMPLY:
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.
With any luck, your ability to upload files quickly should greatly improve.
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.