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Its a square law.
F/4 is (6/4)^2 = 2.25 times as fast as F/6.
The notion of fast/slow originates from photography and is referring to the shutter speed required for a correctly exposed image.
But... here's the catch:
The exposure depends on the amount of light reaching the sensor, which is proportional to the AREA of the telescope objective. Area is proportional to diameter squared.
But F/ratio is a ratio of two lengths = focal length/diameter.
So for two telescopes having focal ratios F1 and F2, the relative speed of one vs the other is (F1/F2) squared.
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