It is possible that depending on the extensions involved and the site design, a performance penalty may occur. If it goes beyond 10-20%, then you got a setup issue, and you should contact support to get you sorted out.
When using the built in cache system, especially the memcache backend, you can actually expect performance increase, as sh40SEF does not need to rebuild the full SEF url each time, as Joomla does, but can get it from cache almost instantly.
Note that the sh404SEF cache can also optionnally be enabled to provide caching for ALL Joomla database operation (ie: non-sh404SEF related), thus further speeding up the site.
Now you have the best wordpress solution for Joomla on the market
better than Corephp
thank you for your great work
Thank you for the kind words Gui. User satisfaction is my number 1 priority and to hear that you are satisfied makes me very happy!