Fairly consistent with Ren being faster than lightning is this feat, in which Ren throws off 50 attacks in less than 0.1 milliseconds.
We don't have many figures here at all, aside from the number of attacks and the timeframe. Because of that, I'm going to have to low-ball just to play this safe.
Firstly, I will be assuming that Ren is swinging his arm about his elbow, as one does when going for quick swings. Secondly, I will be assuming Ren is about as tall as the average Japanese guy and that his forearm and hand length is proportionate for that height (44.5 cm). Lastly, I will assume that Ren is making 180 degree swings and no more. This motion comes out as very mechanical in practice, and he's likely moving a bit more distance than that, but I digress.
Using the forearm and hand figure we can calculate what's likely the least amount of distance Ren moves per swing.
C = (2)(pi)(r)
- pi = 3.14
- r = 44.5
(2)(3.14)(44.5) = 280 cm
Of course, I said we'd be using 180 degrees as our assumption, so the actual distance would be half this, 140 cm.
From here, it's just multiplication and then sticking this into the velocity formula.
Td = (d)(50)
- d = 140 cm
(140)(50) = 7000 cm
v = d/t
- d = 70 m
- t = 0.0001 s
70/0.0001 = 700,000 m/s
Of course, even though this is faster than lightning already it's still likely he's going faster than this. This doesn't account for any acceleration, and the motion here is very minimal and mechanical. But regardless, this is pretty much near the minimum.
Result
- Ren Fuji Slashes Fast - Mach 2060