It just occurred to me that many don't know what "group-delay"
is. It is physically-electronically a time delay from input to output
that may vary depending on the input frequency versus the
magnitude-phase response of the filter within its passband.

Group-delay is defined as the difference in phase delay versus
frequency difference over very small differences in frequency.
Most analysis programs show that as a matter-of-course using
frequency-domain analyses. Using around 50 or more different
frequencies of a linear source sweep input will show the actual
filter time-delay, input to output, that is very close to the real
thing.

73, Len AF6AY