How are objects rendered with a MEMS mirror for AR?
MEMS systems can have resonant frequencies measured in kilohertz, so as long as you don't mind having the mirror continuously moving you scan fairly fast (accelerating/deaccelerating to hit specific point would be much slower or even impossible depending on the scanning mechanism).
The other option is to use an array of MEMS mirrors as in a DLP chip. These are very small, but typically have a frequency of about 32khz. The array then works like a 32000 frame per second, 1 bit per pixel display. PWM is used to encode gray levels.
Edit:  To address your specific questions:

If it takes so long (relative) to move the MEMS, how are such high resolution images able to be created in the new HoloLens 2?

If the 54 kHz you quoted is accurate for the fast axis rate, the scanner draws one line of pixels every 18.5 microseconds, and an individual pixel probably every few tens of nanoseconds.  It is therefore moving quite fast.  Fortunately, 54/2 kHz = 27 KHz (presuming both the forward and backward direction is used to reach that speed) is too high a frequency to hear or it would be quite annoying to be around.

And I would assume the angles will already be sorted so you consecutively display each pixel in order.

Yes, pixels are drawn out in sequence with a MEMS scanner, but it is more complex than that.  The fast axis scanner is at resonance, meaning it traces out a sinusoidal trajectory.  It thus moves very fast in the center of the field and very slowly at the edges, so you must pre-warp your pattern to compensate for the non-uniform rate at which pixels are drawn.