Walter Johnson was a really great pitcher. But it's hard to compare individual statistics of different generations. If for no other reason than Jackie Robinson, the strengths & abilities of modern ballplayers are much greater than they were 80 or more years ago.
You certainly have all sorts of “enhancements“ on the muscle building side for batters over the years, which like anything would have to be accounted for by approach. The percentage of the potential talent pool funneled to baseball as opposed to other sports was also likely lopsided in Johnson’s day compared to the modern era.
Pitchers reply on changes and misdirections, something that is not era-dependent. As for the speed aspect (which in and of itself would not be what made Johnson the greatest, but is at least a measurable), Johnson seemed to be in the mix rather than modern ballplayers’ strength and abilities dwarfing his per some reports and assessment attempts:
https://www.baseball-almanac.com/articles/fastest-pitcher-in-baseball.shtml“At the Aberdeen Proving Grounds he was measured using the ever-popular speeding motorcycle test, once used in 1914 with Walter Johnson who reached 99.7 mph, and Feller reached 98.6 mph. The results of the test from the "new meter" were reported the day after the initial article:
Humphreys' 'Hard' Un' Faster Than Feller's, Meter Shows
CLEVELAND (AP) - Three Boston Red Sox threw a baseball 122 feet a second into a new photo-electric pitching meter yesterday. Three Cleveland Indians could do only 119 feet.
Pitchers were not included in yesterday's test but "unofficially," Bob Feller of Cleveland threw three balls into the meter from a distance of 20 feet. The best mark he recorded was 119 feet. His less-touted teammate, pitcher Johnny Humphreys, recorded 127 feet. There will be a contest for pitchers later.
Jimmy Foxx, Jim Tabor, and Roger Cramer made it a clean Boston sweep with a first-place tie in yesterday's fielders contest.
The best the Indians could do was a tie at 119 feet by Ben Chapman, Julius Solters and Jim Shilling.
Cleveland men who developed the speed meter said the only comparable scientific marks were made in 1917. Walter Johnson threw the ball 134 feet a second, Christy Mathewson 127 and "Smoky Joe" Wood 124. Their speeds were shown by a gravity drop interval recorder.