mod_disk_cache and mod_mem_cache make that pretty irrelevant.

http://httpd.apache.org/docs/2.2/mod/mod_disk_cache.html

re:edit hxincl uses a similar style as ssi but with a couple of enhancements:

http://manpages.ubuntu.com/manpages/trusty/en/man1/hxincl.1....

if you want exactly SSI syntax, ain't nuthin' stopping you from pointing curl at localhost.