A friend of mine created a new website for Grubby, a Dutch professional gamer for WarCraft 3 and StarCraft 2. This PHP website is hosted on a quad-core VPS with 8 GB RAM. During busy moments, the server has a thousand simultaneous connections. During testing, the server had about 600 simultaneous connections, which caused a load of 2.7* on the VPS.
This load was expected, so the website was built to make use of Hiawatha's CGI-caching feature. Each page that is generated by PHP will be cached by Hiawatha when the PHP script allows it. The pages were only cached for a few seconds, but this caused the load to drop to 0.05. Even with more than thousand simultaneous connections!
So, if you have ever asked yourself if Hiawatha is capable of serving heavy traffic website, the answer is definitely: yes!
*) For people who don't know what this number means; it's the number of required CPU's to handle the load. So, a load of 1 on a single core system means the machine is 100% busy, just like a load of 2 on a dual-core system.