Batters facing Walter Johnson often alleged that they could not actually see the ball. Umpire Billy Evans admitted that even he couldn't tell if the ball was crossing the plate or not. Quite an admission.
Batters often admitted that they couldn't tell if they were swinging over the ball, under the ball, or anything. Even Babe Ruth told of his first AB against Johnson in 1915. He says he stepped into the batters box. Bam, bam, bam. Back to the dugout. Easiest victim Walter ever had. Babe never swung, never saw any pitches. But he heard something swish by. He told the ump that the pitches sounded
Another batter, I think it was Jimmie Dykes was, was batting agaisnt Walter and his arm comes down. Jimmie is waiting and the ball never arrives. Then the catcher is returning the ball. Jimmie turns to the ump, with questioning eyes. The ump tells him to take his base. Huh? says Jimmie. The ump then informs him that if he doesn't think the ball clipped him, feel his bill cap.
Jimmie does and the bill is turned all the way around. Jimmie turns white. Never even saw a ball! Only Nolan Ryan was that fast in modern times.
No one ever alleged they couldn't even see a ball. So I equate Johnson with Ryan. Ryan was timed over 100 mph.
Feller was time at 98.6. Body temperature. Many equated Feller with Grove. But no one ever claimed that they couldn't follow Feller's pitches. So I measure Johnson over 100. With Ryan.
When Feller came along in '37, he electrified the BB world, with his 98.5 mph velocity. So that was the top rate for the BB world at the time.