none. Server side includes are done at the server level. This means when the page is fetched, the includes are added THEN it is sent to the requesting IP.

The end result is the page looks no different than a regular static HTML page.

Thanks

One thing to watch out for is that using includes can create a lot of identical content blocks that appear on a lot of pages.

Watch the balance between unique content versus the repetiive includes. You want to make sure your pages don't end up looking "too similar" in Google's eyes.