I bought this and played for a bit. I started with Black Label, B-Power, because I'm very familiar with it.
I began with Stage 5 in training mode. The slowdown seems pretty similar to the X360 version, though it seems to be missing a bit here and there. What's worse though, is I can't get any hits off the laser turrets in the first part of the stage. In the Steam port you can get WAY too many hits here (several thousand more than you should), in the X360 port it was identical to the PCB, but here, if I'm lucky I get a paltry 50-100 hits when I first start a hyper, then nothing. I just sit there and no hits. It's bizarre. Maybe it's just a bug in training mode (I hope so!), but if it's like this then BL is worthless as-is.
Input lag feels OK, I'm sure it's 3-4 frames like the other livewire ports.
This does seem to be based on the Steam port rather than the X360 port.