I'm like, 99% sure this is true as I also remember reading this around the time the Unleashed Project was starting up. In terms of what feels the fastest? Definitely Unleashed, yeah. The sense of speed in that game is absolutely unparalleled and is definitely helped by things like the overpowered-as-fuck air boost. In terms of speedrunning too, there are some tricks that work in Unleashed that aren't in Generations, like the QSS (quick step slide), BJG (big jump glitch), M-speed and D-speed, and these make speedruns of the game incredible to watch and quite technically challenging to pull off. EDIT: As somebody is going to point this out, yes, Generations does allow some of that tech to be used but it's a much, much smaller window of input than Unleashed allows and is rarely consistent enough to be worth it.