Powerconnect 6248 performance problem

We recently brought a MD 3620F (connect by FC to an R720) the network speed over 6248 is slow.

If connecting the R720 by a crossover cable to an host directly the network speed good, so our guess there is something probably missconfigured on the 6248.

We even tried Bonded connections from the R720 (4 Nic bond) to the 6248, but the speed is nearly the same.

So we are seeking for a solution

regards