I talked about this in the OP. Memory maps are sometimes a little faster. See:

    $ ls -l full.txt
    -rw-rw-r-- 1 andrew users 13113340782 Sep 29 12:30 full.txt

    $ time rg -c --no-mmap Clipton full.txt
    294

    real    1.337
    user    0.470
    sys     0.866
    maxmem  15 MB
    faults  0

    $ time rg -c --mmap Clipton full.txt
    294

    real    1.045
    user    0.722
    sys     0.323
    maxmem  12511 MB
    faults  0

But in recursive search, especially when used for lots of little files, they end up provoking substantial overhead that slows everything down.

And this might change depending on the platform.