https://www.youtube.com/watch?v=FfT7w3pIzDg
Feat happens at 2:05. Kabuto and that bug monster have an entire fight and the rain does not move, at all. I'll be measuring how much time is passing per "second" and then assuming Kabuto is capable of moving at least a meter per second in his slowed-time, as he is moving at "real speed."
Rain Speed - 10 m/s
Average Human Walking Speed - 1.386 m/s
Rain Size - 0.006 meters or 48 px
Clock Up Start - 2:16:57
Clock Up End - 2:53:98
0.006/48 = 0.000125 meters
0.000125 meters/10 m/s = 12.5 microseconds
53.98-16.57 = 37.41 seconds
37.41 seconds/12.5 microseconds = 2,992,800x
2,992,800*1.386 = 4,148,020.8 m/s
Results
Kamen Rider Kabuto's Clock Up Speed (Low) - 0.0138c, Sub-Relativistic