Sounds about right, since the speeds being advertised by the ISP are MegaBITS per second and speedtest will likely be showing megaBYTES per second. The quick and dirty conversion calculation is to divide by 10, so 100 megaBITS per second works out to 10 megaBYTES per second under perfect conditions.
As for speeding up wifi, you can't. The MBAir has an 11g wireless card in it and there's no changing that, same with the iMac. Your only option would be to buy a USB 11ac device and then because of it being USB you'd probably only get a marginal increase. You don't tell which model iMac you have, but basically unless it has an 11ac wireless card in it, you're likely looking at 11g speeds all around where you won't necessarily be able to take advantage of MIMO functions of the router and your maximum download speed is 54Mbps, which you'll never actually see because that's the maximum speed on paper under perfect lab conditions and excluding any kind of network overhead.
Welcome to the world of where advertisers walk right up to the line of what they can say without falling foul of false advertising laws and then kind of lean over that line a bit by intentionally trying to make you assume certain things and burying anything that might disabuse you of those assumptions in fine print you practically need the Hubble telescope to be able to read.
We just upgraded our home internet to the fastest fibre connection available, 100 Mbps, but even with a compatible router (Linksys EA4500) speedtest.net shows download speeds of only 5-10 Mbps, and that is sitting right next to the router. What can I do to speed up the wifi?
Using mid 2011 Macbook Air (no ethernet port). Desktop is iMac, but far away from router to hardwire it... and same wifi speed as laptop.