Word of advice for any Quest owners who use Link on a windows10 machine; 
Make sure you turn OFF win10's "Game Mode", what Game Mode does is automatically set non-game non-critical processes to below normal priority, which can stop background things from harming your game performance. HOWEVER, Oculus Link rendering is NOT linked to the game executable and Game Mode will happily put the priority down to the fucking ground, meaning it can look like you're playing a 240p youtube video even though the game in the mirror is playing flawlessly. 

Turn off Game Mode and make sure the priorities for all the Oculus threads are set to normal. If your CPU is an older model setting them to high can actually improve Link performance slightly so it's worth a try.