Attached is a little stress test program. In the command line arguments, you can set the number of watchers, the frequency of file creations and the number of seconds you want the program to run. I've run it with up to 100 watchers and 10 file creations/sec for 30 seconds. There doesn't seem to be any drop-off in performance. This holds for a Core i7 Windows computer as well as a Raspberry Pi 2. You can run it on your hardware to see if the FileWatcher will work for your purposes. I suspect it will.
My preliminary research indicates that this library doesn't use polling on Linux (at least after kernel 2.6.13) but rather native file system events. This is tentative but, either way, the latency between file creations and the corresponding events being raised was often around 1-2 ms on the RasPi2.