Fujifilm X-E2 v Sony A7 Autofocus comparison video showing an AF speed differences between the Fujifilm X-E2 and the Sony a7.
In good light they are pretty much evenly matched – although on close up focusing of my hand the Sony failed a lot more often than the Fuji. Sometimes the a7 is quicker, sometimes the X-E2.
In poor light the X-E2 wins hands down, locking on solidly every time where the a7 would rack back and forth and sometimes take several attempts at the shutter button before it locks.
This is just a very basic sample. The real-world is really where the X-E2 shines. For indoor family stuff the camera is a huge leap forwards compared to any Fuji in the past, including the X100S in my opinion. It appears to use the phase detection into a much lower light where the X100S gives up with that pretty early on.