fast lenses for NX100

I use an NX100, own the 50-200mm and have two kids in soccer, so I feel qualified to comment. 

Continuous AF on this camera is not helpful.  In theory, it constantly adjusts focus so it will be ready when you press the shutter release.  In practice, it eats up your battery and still does a fore/aft focus hunt when you release the shutter.  I use the AF S setting.  Still, while the focus is pretty fast, it's just not fast enough for kids running toward you.  Mirrorless cameras have come a long way in the last few years, and the AF on new models is words apart from the NX100.  If you're sitting on the sidelines and shooting as they run past, you'll probably be fine.  If you're at either end of the field shooting as they run toward you, you'll have to work harder.  I find it useful to either prefocus on an area where the kids will be in a fraction of a second, then wait for them to arrive, or choke down the aperture and get a wide depth of field.

In practice, it's mostly moot for me.  My older son plays on bigger fields, and I didn't get enough subject isolation with the zoom lens.  Now I shoot a set of fast primes depending on how far I am from the action.  I have Minolta MC 300mm f/4.5, MD 200mm f/4, MC 135mm f/2.8 and MD 100mm f/2.8.  Still, this is not as flexible as the zoom lens, and I pass on a lot of shots due to distance.  I usually use each lens for about 15 minutes so I'll have a variety of shot types to choose from later.

And, I've found, the moments you want to catch are usually over before you can lift the camera.  Even instant AF wouldn't help then...  Just shoot a lot, and improve your odds of getting 'the one'.  That's part of the beauty of digital photography...