I'm currently hosing a couple low-traffic sites from my server, and one of them uses a CGI program that I made to decrypt 1024 bit key RSA data. Some client-side script encrypts it. This way, people can make online purchases without me needing to get a dedicated IP, or SSL certificate. Saves me money! And the (shared) server seems to preform just fine. Are you suggestion that I'm pushing my limits by doing this?
On the lower end, shared hosting is indeed just not enough. Good for low bandwidth requirements static websites. And that's it. As soon as one enters into PHP, ASP.Net, Ruby on Rails, or any other server-side scripting solution the need to have your own modules installed and otherwise configure your web server to your tastes and needs is just incompatible with that hosting plan.