I just finished putting together a “quick” video showing how the LongMill 30x30 performs under load. It’s 10pm on the Friday of a long weekend but hopefully it all makes sense. For regular use (just under 3kgs load) this ends up at around ±0.13mm (±0.005") in the x-direction and ±0.2mm (±0.008") in the y-direction.
In the x-axis, the movement comes from a small amount of looseness in the linear guides, the delrin v-wheels, and a small amount of backlash. In the y-axis, the majority of this movement arises from twisting in the long x-axis rail as well as a tiny contribution from the delrin wheels and backlash.
It’s hard to compare these results to other similar CNC routers since documentation like this is hard to find from other companies: if you do know of any then please contribute in the comments. Otherwise, I’d speculate that this testing is easily comparable to any other in-class machinery out there.