How bad is the speed hit for SSI really?

The DH Wiki and elsewhere note that there’s a small speed penalty for running an HTML file through the SSI parser. I’m trying to figure out how bad that hit really is–are we talking negligible relative to the overhead of an HTTP request, or is it actually significant?

I’m curious because I’m considering adding a small bit of rotated content to a large number of static pages, and I always like to keep things as efficient as possible in terms of server load and such, both to be a good shared server citizen and to improve end-user experience. Better able to deal with traffic spikes is nice, too.

So really I’m wondering if the SSI parser is going to be better or worse than the additional HTTP request if I used an iframe for the content instead (or, for that matter, if either is bad enough to make it worth regenerating all the pages regularly to keep them 100% static).

Anybody have hard numbers on this? My Google-fu was weak other than generalizations of static > SSI > PHP, with additional HTTP request in an indeterminate position.