google gives only approximate piston speed calculations, so I dived into my old books.
Vpiston = 2 π n r (sinφ + (r/2l) sin2φ)
in which n = engine speed, r = half stroke, φ = crank angle (in rad), l = rod length.
However small the influence of the rod length may be, it's still there. Longer rod means more even piston speed, shorter rod means more variation in piston speed (and thus a higher max speed since the min speed cannot be smaller than zero).