CGI's performance may have been fine for Perl, which has very low startup overhead, but those who wanted to write apps in most other languages needed something

CGI's performance may have been fine for Perl, which has very low startup overhead, but those who wanted to write apps in most other languages needed something else. And even then there was mod_perl, as you mentioned, which eliminated this overhead for Perl as well, suggesting that perhaps CGI performance was often not fine.