Koufax's MPH was measured and he clocked in at 93 MPH. There were several players who were measured at the same time as he was and he was not the fastest among his peers.
There is a more comprehensive list of that test, but this Sports Illustrated article from when Ryan was clocked at over 100 alludes to Koufax's speed.
https://vault.si.com/vault/1974/09/1...p-for-an-angel
One poster above made a very wise distinction about what defines 'best', when he said that a career value and peak value can give two very different answers, and both be correct.
There is no way Koufax can be viewed better than Warren Spahn based on their careers. Simply no way.
However, if you are talking about who was better in their peak years, then Koufax is going to be the answer.
I do want to point out that many do cite Koufax's early retirement as a way of giving him more credit toward someone like Spahn. While that is a good 'what if' scenario to take into consideration, keep in mind that Koufax is not the only player that has strong 'what if' factors such as that. Spahn himself lost three years due to WWII.
Had Spahn not lost three years, then he is easily a 400 game winner. Wins are not the best indicator of a pitcher, but he still lost three years of production. Any way you slice it, add those three years into his career, then his lead on Koufax is impossible to overcome.