On most context involving "using grep", shell scripts, interactive shell, etc, it's nothing.

81 Vs 128 system calls on my laptop (just printing --version)

    $ strace -fc grep -E --version 2>&1 | tail -1
    100.00    0.000000           0        81         3 total
    $ strace -fc egrep --version 2>&1 | tail -1
    100.00    0.000000           0       128         6 total

Measured with "time" 0m0.002s Vs 0m0.003s, always, testing 4 times each option.

Most situations where it could be being used, probably are surrounded by much bigger optimizations to work on.

Good point. Though invoking $SHELL and parsing the script (as short as it is) can't be cheap either. Academically speaking of course; I'm not trying to justify the change on performance grounds.