Hi,
Each time you start a web process, it uses a sizeable chunk of resource overhead.
Every server-side include (SSI) in a page requires a new process (like above) - try to keep the number of SSI's in a given page down to a minimum.
On the other hand, a single SSI can do a pile of grunt work (including HD accesses/writes) without even warming up your box (if it's unix/linux).
I use SSI extensively and without any performance issues (that is, after I distilled 13 SSIs/page down to 2 or 3
)
If you need help with SSI strategies, let me know - I'd be happy to help...
-Dino