i do not know the plugin speedtest, but i think ittests your internet speeb by downloading an file from an server. Even if your internet-connection is 1GBit in downstream, the server could be busy, so this steed is more or less just an hint.

You should test the speed on an local file transfer. you should mount a network-share with nfs or smb and copy a file on ths share. If you use rsync to copy, it can show you the current speed, when you pass the "--info=progress2" parameter to rsync.

Do you have a GBit patchcabel installed ?
You can check via telnet

install this :
apt-get update && apt-get install ethtool


Then check :
ethtool eth0

The box is connected by 100Mbit
check your switch / router / cable ....

Put the cable in the lan-port on your router were the pc is.
Some router have ports for 1000mbit and for 100mbit or you can config it.

it is not a problem of your Dreambox.