You might not even get a perceptual performance boost at all. I recently upgraded from a 2013 Moto G to a current-gen Nexus 5x and it feels about the same, maybe even slightly slower, in terms of latency for common tasks and how often it gets bogged down. I think part of the problem is that the new phone has a higher-res screen, which some apps (like Facebook) adapt to by shipping much larger icon sets, loading higher-res images and video, etc., which negates any performance improvements. The camera is definitely nicer, though.