However, with SSI, the server must first read the SHTML file, process any directives (which can also be program output or files to be merged in) and then deliver the fully assembled page to the reader. Whew! That is a lot of work.

If you are expecting thousands of hits per minute, you might want to use SSI sparingly. On the other hand, keeping your pages trim will more than make up for the SSI performance cost. (hint-hint) In a typical setting, the productivity gains you will enjoy will greatly outweigh any potential performance problems.

Today, horsepower is cheap and getting cheaper. When making a decision on the value of SSI and you have busy site, do give some value to your time savings vs. the cost of a faster server.