We propose a fast quasiadiabatic approach to the design of optical waveguide devices. This approach distributes the system adiabaticity homogeneously over the device length, thus providing a shortcut to adiabaticity at a shorter device length. A mode sorting asymmetric Y junction is designed by redistributing the adiabaticity of a conventional linearly separating Y junction. Simple procedures for the design of fast quasiadiabatic devices are outlined, and the designed Y junction features large bandwidth at a shorter length than the conventional linearly separating Y junction. The proposed device is verified with beam propagation simulations. A mode conversion efficiency of larger than 99% is observed for the designed Y junction over a 220 nm range.