It would probably help if you clarified what you are measuring and how you are measuring it ...
There have been some reports of radio interference between the SS 3G and wifi on home networks - normally resolved by positioning the SS and wireless router further apart. This won't be an issue if you are using a wired connection for your measurement.
Otherwise the SS will always consume some bandwidth but shouldn't consume that much - 64Kbps each way per active call, though I assume you mean when there are no calls active? Beyond that, it polls constantly for incoming calls (350-400bps) and occasionally updates its configuration which will create competition for other network users while it is downloading - but only while it is downloading. If this were the cause of the 'slow down', then switching off before it had finished and only on again occasionally would pretty much guarantee that the problem would persist - far better to switch the SS back on a bedtime, let it get its updates done overnight and leave it on permanently.
Other things to check would include whether your mobile devices are making data calls in the background - to synchronise calendars, receive email etc. Always better to use wifi rather than 3G for that.
Philip
