Problems with conversion time using K8055 under XP

At our school we use 12 of your USB Experiment Interface Boards K8055 under VisualBasic 2005 for educational purpose. These boards are connected with notebooks (WINDOWS XP).
We have a certain problem concerning the velocity during switching between Output and Input. The conversion time is round about a quarter of a second. This is far too much for our jobs.
We copied your source code (Demo PC soft VB_net from the included CD) and replaced the k8055d.dll by the latest one, but the boards don’t work better.

When we use the diagnostic software (downloaded from the Velleman website) however the board works as fast as expected (conversion time 20 ms).

An interesting detail is that an old pc under WINDOWS 98 doesn‘t show any problems. For this reason we suppose that our problem is caused by the USB-Port 2.0 .

If you could help us to solve our problem, we would be grateful.