My gps source is a cambridge 302A. I run XCSoar 5.2.4 on an old iPAQ 3850. It is stone-axe reliable, and has the brightest in-sunlight display I've ever seen. I also have a Dell Streak, but the display is really nowhere near as good as the iPAQ. However.... the iPAQ won't run later versions of XCSoar gracefully.

I've had exactly the same experience as you except I think I can see the Dell Streak screen much more clearly in polarized sunglasses than the iPAQ 3850. In the middle of the contest I upgraded to XCSoar 6.6 Beta2, and only during flight I noticed much slower responses. For example, just before arriving at a turnpoint the XCSoar would freeze for 30 seconds or more, and unintentionally I would make the turn later than planned. Then on a final glide the info boxes would update once every 30 seconds or so, which made me quite uncomfortable too. The same day I reverted back to an older version (I believe it was 6.5.2) which works fine. I did not post a ticket because I did not see anyone else complaining about it.

Wiktor