my site takes ages to load without www. prefix

gpstudios.com hangs for about 5 - 10 seconds www.gpstudios.com loads instantly.

I've got it so it redirects to www.gpstudios.com if its missing for URL consistency.

What's going on? Must be something wrong with the DNS setup, but it succeeds all DNS checks with external DNS testing services (eg - http://ift.tt/1pRGDL7)

How do I fix it?