Playing around with gaits by directly editing stuff in gui/gm-editor I was able to get various humans/dorfs/elves/gobs to cap out at 6.666 displayed speed.
I was able to get a couple of glitchy moments like when I accidentally made an elf something like 30% normal size and the displayed speed was like 7.420 I think, and you can directly change a couple of different things and temporarily get the same displayed speed but it goes away if you change direction, returning to the cap of 6.666.
From the comments in the raws it looks like a speed of 1 tile per frame would be 8.100 but I've yet to actually get a creature capable of this speed, though I haven't tried non-humanoids, I have no idea what happens if you were to go faster but I assume your character implodes or turns into a wambler which then implodes or something like that.
Things I messed with were all under the body header in gm-editor.
I think body plan has the sub-headings size info and gait info, gait info is obvious enough, first heading is walking, with speeds in descending order, second is crawling, then climbing and swimming (or swimming and climbing, I forget), and I think flying is last.
I think there is a "sweet spot" with body size and stuff, I noticed that if I went too far either direction I'd get slower, I think a default elf caps out when the body size is 3900~4000, didn't check as closely with the other two races, and there is a bit of a speed modifier due to the values under uh, body material percent or something, the headings were like total, base, muscle, and fat, with values around 65235 for a human base as I recall? Those changes only come into play if the top flag under a given gait is true (layers_slow), otherwise being stacked with muscles and belarded with fat doesn't matter for speed.
These were all around when I was trying to make a human adventurer expy of Saitama from Onepunch Man, who is too strong and tends to end fights in a boringly immediate fashion with a single blow.
Later I noticed the rather hilarious nature of the "brawl/non-lethal/lethal/no quarter" system whereby if you add edge tags to your barehanded attacks and adjust the contact/penetration perc values enough to enable you to delimb/behead targets you can run up and chop someones arm off but still have non-lethal responses since you're "unarmed"... which is almost ironic.