Greetings! I just got my 625xt today and love it so far. But I have to say that the 'pace' display gave me quite a surprise. Then I found this forum/thread.
I ran today on the road and whenever I came to a stop waiting for road cross signal, the pace value shown will gradually (and really slowly) increase while I am not moving at all. It looks like it is averaging over the last several minutes.
I too, wish that the pace display could be more instant. Averaging over last 10, 20 or even 30 seconds should be fine but minutes seems too long. I understand that GPS technology is not at the accuracy level that can give true instant pace value but from the track log in Gym Pro it seems that the device samples GPS location with a 2-second interval most of the time. 20s then is enough for 10 sampling points and should give a good pace/speed calculation.
The old GPS watch I used has a 'pace average' option that let user choose how much pace averaging is done and it works rather well ( I always use 'average less' though).
Also as Vincent pointed out, the 'speed' is rather instant already. In its simplest form the 'instant pace' could be just (60/speed), however the speed is calculated now. This should be trivial to do.