The reason your connectiion appears to slow down is that your ISP provides a certain amount of bandwidth to your connection and this is divided among all the computers connected to it.
So you have, say, a 5Mbs connection, then one computer gets the whole 5Mbs to itself, 5 computers it's 1Mbps per computer. It's not quite as simple as that, it depends a lot on what the various users are doing, but that basically is what is happening.
Unless you have a very fast internet connection, the limiting factor is the speed of your internet connection to your ISP.
Connecting by wireless, as opposed to ethernet, is almost always slower, although the reduction may be very small. It depends on a lot of factors which include : distance from the router; structural materials between the computer and the router; alignment and positiioning of the various aerials.
For computers with reduced browsing speed using wi-fi, probably the biggest difference you can make is to play around with the aerial position. As an example, I use a USB dongle on my desktop which is mounted using a magnetic clip and a short extension cable at the very front top of the right hand side. if the mounting clip slips down by as little as half an inch ( 1.25 cm ) I lose about 25% of my signal !