Guys went Mach 6+, almost Mach 7 in the X-15, and astronauts go faster to achieve escape velocity. What can harm a person is not speed per se, but sudden acceleration or deceleration. For instance, SpaceShipOne and some tank cannon projectiles both do between Mach 3 and Mach 4. Accelerating like SpaceShipOne doesn't harm people. Accelerating like a tank round would turn people to jelly!
Absent a sudden onset, there's no physiological limitation to how fast you can go (inside something, obviously).
d.o.l.
Criminal Number 18F
Interesting.