A very common issue. Seen this with the unbranded bluetooth GPS's sold on eBay in the past. Where did you get yours? I use the
Emtac Crux II. Cost nearly 40% more than the ones sold on eBay. I've tested mine as a passenger on a Dornier 328 (high wing!), and after the initial difficulty of aquiring the satellites my GPS was coping with over 350KTS!
First thing to remember is this has nothing to do with the power of your IPaq's processor speed. The vast majority of GPS units output a GPS sentence known as the $GPRMC sentence. E.g:
$GPRMC,092204.999,A,4250.5589,S,14718.5084,E,
350.00,89.68,211200,,*25
This contains the speed at which you are travelling at. Therefore the speed is calculated by the GPS and not your IPaq. The other thing to bare in mind is that just because you are travelling faster does not mean there is more data to process - it's actually the same. Your GPS probably returns a new fix every 1 second, it does this when stationary and when going along at 150KTS (as long as the right number of satellites have been aquired).
Thus the culprit is probably the GPS device (not able to aquire a fix at speed). You can probably prove this if you
connect your IPaq up to MS Flight Sim 2004 
(with FS acting as the GPS device). You'll see your IPaq can cope with speeds over 700KTS