Plain and simply put, Ubuntu will get faster speeds than windows. It was designed to get every last bit of bandwidth and suck it down (You can even steal it from others on the same network, but anyway...).
Windows reserves some bandwidth for things like windows update, etc. and there are many different applications that use the internet at random times: Updaters, etc.
However, sometimes a slightly older version of a wireless adapter software can be used to browse the internet a bit quicker, as I've noticed. You might try this out. Search up ways to speed up internet speeds, but be careful, because not everything that claims to increase speed will, nor is it always good for your computer.
The less 'automatic' programs you have running, likely the faster (if slightly) speeds you will get.