Go to Settings > Connectivity > Location and deselect enable underneath Network positioning then press save.

Edit:I've had one comment on Bugzilla, this did not work for them.
Can people post in this thread stating whether it worked or did not work for them and under what circumstances it worked/didn't work.

My N900 got a fast fix under clear skies and my desk near the window, the GPS tracking is near-enough real-time.