with only one online user visiting the same page, here is my finding:
when sh404sef disabled: server load around 20-40%
when sh404sef enabled: server load up to 70-99%
enabled joomla debug mode to find out that when sh404sef enabled, quries run one a page will be doubled compared to sh404sef disabled.
sh404sef does add some overhead to the Joomla SEF system, but usually around 25%. The number of queries added depends on many factors, including whether you activated the url disk cache and, most importantly, on how the 3rd-party extension support plugin is done.
sh404sef does not have any support for JReviews builtin, which means you are either using JReviews router.php file, or a sef url plugin provided by somebody else. With that kind of overhead, I would certainly review the way this plugin was written, and probably try to optimize it.