Problem whith my Internet connexion speed.

Hello,
I have an internet connection with a 30mo / sec throughput

Unfortunately, connecting me via my UTM 9 I do not exceed 600K / sec, while directly behind my modem I have a 3.1MB / sec.

my configuration is as follows
Machine with 2 heart 2.00GHz AMD
3 GB of RAM
2 100MB card networks

Proxy is in transparent mode. 1 Nic is plugged into the modem, 1 nic connected to a WiFi Access Point Cisco WAP4410N.

IP Addressing:
Nic to Modem: 192.168.0.200/24
Nic to Wifi: 10.0.0.1/24

I do not understand why I can not have my speed was 30 mo via Sophos UTM 9