slow TCP speed through linux ip forwarding

Hello,

i hope somebody has an idea for my problem and may point me to the right
direction. Alle servers run kernel 3.18.14.

My problem is that i can't archieve more than 20Mbit/s using a single
TCP stream and i've no further ideas how to solve it.

reduced Network Map

Server A
  |
Linux GW Server
  |
Brocade Router  - Server B
  |
Internet
  |
Server C

The Linux GW Server does nothing more than being an ip forwarding GW.
(no iptables / packet filtering).

I've done the following Tests while the Linux GW has a traffic of around
500-600Mbit/s:

Download file from Server A to C: 10-20Mbit/s
Download file from Server A to B: 100Mbit/s
Download file from Server B to C: 100Mbit/s

4 parallel downloads from Server A to C: 80-100Mbit/s

Same test with no other traffic on the Linux GW:

Download file from Server A to C: 100Mbit/s
Download file from Server A to B: 100Mbit/s
Download file from Server B to C: 100Mbit/s

4 parallel downloads from Server A to C: 100Mbit/s

Same test with net.ipv4.tcp_congestion_control=bic on Server A:

Download file from Server A to C: 70Mbit/s
Download file from Server A to B: 100Mbit/s
Download file from Server B to C: 100Mbit/s

4 parallel downloads from Server A to C: 80-100Mbit/s

ehtootl -S shows no errors for eth0 and eth1 on the linux gw.

I've no further ideas.

Thanks a lot.

Greets,
Stefan