I've had it be horribly slow, wickedly fast, and also absolutely fail to work at all.

The wickedly fast method seems to involve the two devices forming an ad-hoc WiFi network -- just for themselves. 802.11ac/ax is pretty darned fast when there's no competition, on a network with exactly two nodes, and when those two nodes are separated by only a couple of feet.

IIRC, this is not the default mode of operation.