So looking up a set of values (say IP address) from somefilesource in a bunch of logs (300 files with ~ 500k lines)
$ wc -l Some*
$ time fgrep -hf SomeFilesource *log* > Wantedlogs
time grep -f SomeFilesource *log* > Wantedlogs
That is quite a speed up. From now on it's fgrep by default... Now if I need to see whether the --mmap optimization speeds up subsequent [f]greps.