That resonator is perfectly capable of running anywhere between 15.5 and 16.5 MHz maybe even wider. I know very little about fundamental electronics, but I do know how to generate reliable timing down to about 2.5ns.
But here's the cool part, don't listen to me, test it for yourself. 2 machines identical except the resonator, shared signal ground and power source, same source code, and a single button connected to both machines. Do a couple billion things like taking random readings from the analog pins then do hashes with them. Just be sure not to use constants for the math or the compiler will optimize them out.
I'll bet you a glazed donut that they never finish at the same time.