I bought a directional Wi-Fi antenna that plugs into my USB port. At one campground, where Firedude was camp host, his RV was about 800 feet from mine, and I was able to get a signal by placing my antenna in a zip lock baggy, tie wrap it to my refrigerator roof vent, and then run a 6' extension USB cable to my computer.
Basically I had the antenna pointed towards his fifth wheel, and he had a satellite antenna that acted as a hot spot for the whole campground, with his permission of course.
It was a Hawkings brand antenna. It was also back in 2007, so I am sure that in the past 8 years external antenna's have improved greatly, as well as internal antenna's have greatly improved as well.
Signal strength will not improve your access speed, but it can lack of signal will slow it down. So if there are 20 others sharing a 1Mb/second line, you will not get to much faster, but if your internal Wi-Fi antenna strength is weak, it will surely help to use a external antenna.
For the next couple of days try using the computer outside. If that improves the signal strength and thus speed, then an external antenna that you can hang out the window will really help you, as your UV reduction coating on the glass might also be reducing the signal strength through the walls and glass.
Good luck,
Fred.