I'm not sure if this is widely known, I couldn't find any reference to it when I had problems.
I was about ready to junk my sat nav application as I couldn't be waiting for 15 minutes for a GPS lock on my XDA Serra. I tried all the 'fixes' that are listed on here and other places, got all flavours of QuickGPS that had been ripped off the Stellar and other devices and none would work.
I happened accross the O2 Serra site and found a link buried on their site, away from the glossy branded pages (kinda like a web-cellar if you will) marked 'GPS fix'. It needs you to input your serial number, but basically it downloads an exe which when run loads a bespoke QuickGPS to your device. Bingo! TTFF is now almost instant and makes the device usable, just like my old Stellar.
It's still a bit laggy and sometimes I can see the GPS lock dropping off and coming back, but at least it comes back!