I had the power steering drop out going into a turn (fuel pump failure IIRC), and it could have easily caused an accident (I physically couldn't turn the wheel enough to complete the turn).
If power steering fails in speed, it doesn't really matter - the power steering does very little when the car has some velocity. If you can't turn the wheel at speed, then there's something else than power steering pump failing (the steering must have been locked by something else).
Or you have very serious muscle weakness, at which point I'm not sure if driving a car is a smart option in any case.
I expect the total force required to turn depends also on the vehicle's weight, degree of turn, etc. My manual, 90's Japanese sedan (approximately 3,000 lbs iirc) stalled on me while I was exiting a freeway at ~30mph, along a round, ~270° turn. My first instinct was to try put the car back into gear, but before I could effectively do that, I realized I needed both hands on the wheel just to wrestle the car through the turn. I could have easily caused an accident. The disorientation of losing power steering while turning, and having to move my right hand from shifter to wheel made this a serious situation. If my car were heavier, or the turn were tighter, or if I reacted slower, (all of which are more likely than a very serious muscle weakness), it could have been much worse.
I had the power steering drop out going into a turn (fuel pump failure IIRC), and it could have easily caused an accident (I physically couldn't turn the wheel enough to complete the turn).