More seriously than my flippant reply last night, there are several other things that are relevant.
First, I assume you are on FTTC rather than ADSLx. The loss on that by using an extension for the modem/router depends on many factors, as MrSaffron says.
For instance, whether the computer is connected to an ethernet socket on the router or wirelessly. We could also do with knowing if you are talking about the connection speed, (sync or rate), or speed test/download speeds. Plus the expected and actual speeds.
Other questions follow from the answers to those.
My broadband basic info/help site - www.robertos.me.uk.
Domains, site and mail hosting - Tsohost
Connection - Three 4G, tbb tests normally 35-45Mpbs down, 65Mbps off-peak, 9-24 up.
If you never think of anything off the wall, you'll never think of anything original.