The 50mm does not has USM autofocus. It has the older and much slower microdrive autofocus system.
In order to get fast AF, you need a lens that has RING USM. I captalize 'ring' because there are two types: ring and micro. The ring USM is the newer kind and it also allows for full time manual focusing, meaning that you can move the MF ring even when the lens is set in autofocus mode, so before buying a lens I would do some research on what kind it has.
The lenses that I would reccommend for basketball are the
Canon 50mm f/1.4
Canon 85mm f/1.8
I haven't used the 50mm, but I can say from experience that the 85mm is VERY fast at focusing. It is also razor sharp. The downside is that on a crop sensor, which the XS is, the 85mm is approx. the same focal length at a 135mm lens on a full frame camera meaning that it might be too close in most gyms.
I also have to say that the Canon XS is NOT built for low light sports, or pretty much any object with changing directions and velocities. It has a relatively "simplistic" 7-point AF system which sometimes struggles in Servo mode because all the sensors have to calculate SO much data, while, on the other hand, the 1X series of cameras with their 40+ AF points have it "easier" because they have several AF points that can be activated simultaneously to achieve focus.
Add the entry-level AF with the cheap and slow 50mm f/1.8 and sports photography becomes considerably more difficult unless you manual focus.
I hope all of this makes sense. And I'm sorry for all the punctuation. And I'm sorry if it seems like I'm bashing these products. I've gone from using a canon XS and the 75-300mm lens, which has the sam AF as your 50mm, and now I wonder how I ever got a decent sports photo at all. Heh....