Very slow network on RHEL 6 guest (Hyper-v)

Hello!
Hyepr-v (2012 R2), 6x RHEL guests.
On all guests network speed 100 Mbit/s (Check that by iperf, lftp and speedtest.net script)
But on one machine (Web-server) i have very slow network. Link speed 100 Mbit/s, but real tranfer speed not reach 15 Mbit/s.
Network adapter is "Legacy network adapter".
What happened?

On slow server:

[root@some_host]# dmesg | grep "eth0"
eth0: Digital DS21140 Tulip rev 32 at MMIO 0xfebff000, some-mac-address, IRQ 9.
eth0: Using EEPROM-set media 100baseTx-FDX.

[root@some_host]# iperf -c 192.168.local.ip -d -t 60 -i 10

Client connecting to 192.168.local.ip, TCP port 5001

TCP window size: 19.3 KByte (default)

[ 5] local 192.168.host.ip port 53633 connected with 192.168.local.ip port 5001
[ ID] Interval Transfer Bandwidth
[ 5] 0.0-10.0 sec 12.8 MBytes 10.7 Mbits/sec
[ 5] 10.0-20.0 sec 13.8 MBytes 11.5 Mbits/sec
[ 5] 20.0-30.0 sec 19.0 MBytes 15.9 Mbits/sec
[ 5] 30.0-40.0 sec 12.4 MBytes 10.4 Mbits/sec
[ 5] 40.0-50.0 sec 12.1 MBytes 10.2 Mbits/sec
[ 5] 50.0-60.0 sec 10.6 MBytes 8.91 Mbits/sec
[ 5] 0.0-60.0 sec 80.8 MBytes 11.3 Mbits/sec

On any other server:

[root@other_host ~]# iperf -c 192.168.local.ip -d -t 60 -i 10

Client connecting to 192.168.local.ip, TCP port 5001

TCP window size: 69.6 KByte (default)

[ 3] local 192.168.other.server port 49551 connected with 192.168.local.ip port 5001
[ ID] Interval Transfer Bandwidth
[ 3] 0.0-10.0 sec 112 MBytes 94.4 Mbits/sec
[ 3] 10.0-20.0 sec 112 MBytes 93.7 Mbits/sec