Install iperf client on windows from:

https://iperf.fr/

or

Ubuntu using:

sudo apt-get install iperf

Try the following from the client machines inside the network and/or from the Smoothwall:

(using speedtest sites through the Smoothwall are not recommended)

iperf -c iperf.securityinspection.com

iperf -c iperf.saratov.ertelecom.ru

iperf -c st2.nn.ertelecom.ru

iperf -c iperf.ips.versatel.de

iperf -c iperf.testdebit.info

iperf -c iperf.cds.com

iperf -c iperf.lan.kth.se

(More servers can be found here: http://en.wikipedia.org/wiki/Iperf)