Bessel filters are optimized for maximally-flat time delay (or constant group delay). This means that they have linear phase response and excellent transient response to a pulse input. This comes at the expense of flatness in the pass-band and rate of roll-off. The Bessel filter is only available with low pass selectivity.
Group delay is a measure of how long it takes a signal to traverse a network, or its transit time. Constant group delay in the pass band, well into the transition band, indicates a very good step response. Group delay with peaking indicates overshoot and ringing. The group delay of a filter is nearly proportional to its order. Also, filter group delay is inversely proportional to filter bandwidth (small percentage bandwidth filters have large group delay). From the bode plot below, it shows that the attenuation beyond the –3dB cut-off frequency is not as steep as Butterworth.