The game calculates how long it takes a given resource extractor using more than just the speed of the extractor. The formula is:
(mining power - ore mining hardness) * mining speed / mining time = production rate in resources per second. You can verify that on the wiki at https://wiki.factorio.com/Mining
An example of this in the baseline game:
Iron ore has hardness 0.9 and mining time 2. An electric drill has mining power 3 and mining speed 0.5. Plug that into the above and it's (3-.9)*.5/2 = 0.525 ore per second.
However, stone is slight different with 0.4 hardness instead of 0.9. That change results in 0.65 stone being mined per second.
