My old desktop parses 4.3 Gb in less than 2 minutes (without doing much useful, than is), so I guess there's optimizing potential. Less than 0.1% memory is used by that process.

$ time perl -w
use strict;
my $FH = undef;
open($FH, "<", "a.dvd.iso") or die "bah";
while (<$FH>) {
  print "hoho" if m/^ho/;
}
close $FH or die "doh";
__END__
hohohohohohohohohohohohohoh...
real    1m40.487s
user    0m32.321s
sys     0m5.729s
$ ls -lh a.dvd.iso
-rw-r--r--  1 user user 4.3G Oct 19 07:51 a.dvd.iso
[download]