It is currently single-threaded (at the time of writing, v4.4). The main thing affecting its speed is the complexity of the shaders that you feed into it.
If you know how to export heightfields using a plan view render and luminosity, I say go for it. Just keep the micropoly detail close to 1 (or above) so you don't lose any detail. The micropoly detail controls the polygonal model that is being captured, so you want it to be not much less than 1.
AA 2 should be good, just make sure that the Pixel Sampler shows minimum samples per pixel is at least 1. You might want to change it to non-adaptive (max samples).
If you crank the micropoly detail above 1 (and AA is 2 or more) you can get a better quality version than the Heightfield Generate, because that samples only once per pixel.