Fast and slow are old photography jargon. In cameras a lense with a smaller f ratio (f1.4) effectively "grabs" more light in a given time and allows for shorter exposure times but a shallower depth of field, hence you use a "faster" shutter speed. Conversely the larger f ratios (f15) do the reverse and are often used in macrophotography where depth of field is ultra-important.
In telescopes the shorter focal lengths, f5 and below, allow for wider fields of view BUT as they have to bend the light more to achieve focus you pay the price by needing better quality eyepieces, better collimation. Longer f ratios, f6 and above, are easier on eyepieces and the collimation is not as critical but, of course the better you set it up the better views you get.
Hope this helps.
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