Hello Sebastion,

so the rndis configuration works...

No I see on the target with the rt-preempt patch this kernel message:

[   97.206853] g_ether gadget: high-speed config #2: RNDIS  <- the debian host is already connected and we have a putty connection to the target
[  247.216366] g_ether gadget: rndis response on err -22        <- the debian host say:  communication error
[  248.974081] NOHZ: local_softirq_pending 02
[  248.974135] NOHZ: local_softirq_pending 80
[  248.984213] NOHZ: local_softirq_pending 182
[  249.010278] [sched_delayed] sched: RT throttling activated
[  249.996113] NOHZ: local_softirq_pending 02
[  249.996647] NOHZ: local_softirq_pending 82
[  249.996798] NOHZ: local_softirq_pending 102
[  250.986077] NOHZ: local_softirq_pending 100
[  250.987073] NOHZ: local_softirq_pending 102
[  251.976188] NOHZ: local_softirq_pending 02
[  253.286567] pch_udc 0000:05:02.4: pch_udc_ep_clear_nak: RxFIFO not Empty
[  254.972880] NOHZ: local_softirq_pending 02


On this situation the target is very slow. 
top shows:

Mem: 29972K used, 464160K free, 0K shrd, 0K buff, 8040K cached
CPU:  0.1% usr 71.8% sys  0.0% nic 11.0% idle  0.0% io  0.0% irq 16.9% sirq
Load average: 4.52 2.48 1.06 2/86 203
  PID  PPID USER     STAT   VSZ %VSZ CPU %CPU COMMAND
   61     2 root     SW       0  0.0   0 20.1 [irq/18-pch-dma]
   78     2 root     SW       0  0.0   0 16.6 [irq/18-ehci_hcd]
  188     2 root     SW       0  0.0   0 14.5 [irq/18-pch_udc]
   82     2 root     SW       0  0.0   0  9.1 [irq/18-ohci_hcd]
   81     2 root     SW       0  0.0   0  8.3 [irq/18-ohci_hcd]
   80     2 root     SW       0  0.0   0  7.8 [irq/18-ohci_hcd]
    3     2 root     RW       0  0.0   0  6.6 [ksoftirqd/0]

I hope that is right information to find the problem.

Greetings
Alexander