-Ax and -Bx and -Cx make things significantly slower.

[petdance]

I was searching for something like:

ack -A10 'sub foo'

An -A10 takes longer than -A1, but not much. -B is even worse.

$ time ack nonexistentstring
real    0m3.814s
user    0m3.622s
sys     0m0.177s

$ time ack -A1 nonexistentstring
real    0m8.867s
user    0m8.684s
sys     0m0.175s

$ time ack -A10 nonexistentstring
real    0m9.004s
user    0m8.800s
sys     0m0.202s

$ time ack -B1 nonexistentstring
real    0m9.586s
user    0m9.371s
sys     0m0.215s

$ time ack -B10 nonexistentstring
real    0m9.522s
user    0m9.318s
sys     0m0.200s

[petdance]

This was released in 2.16.