I know of one case where a badly written (that's probably the reason) shell script was replaced by a Perl script with a speed gain of a factor of 1275. Not per cent! YMMV
If your shell script uses a lot of "expensive" external commands, may be even in a loop and you can replace that by some internal commands of Perl, the speed gain would be enormous. However, if you "machine translate" such script into Perl by calling exactly the same external commands (through backticks or system), you will be worse off than before.
CountZero
"If you have four groups working on a compiler, you'll get a 4-pass compiler." - Conway's Law