Unfortunately the original issue is still unresolved.
From WP Slimstat 2.5.1 to 2.6 there is still a 30-50% impact on page load times than with all versions prior to 2.5.1. This occurs on a WP3.3 setup or prior WP version. Whereas WP Slimstat 2.5 works fine on WP3.3 and prior.
As awoz stated above, it may be an issue with the tracker, as when a filtered user views the same pages, the impact on load time disappears (as it does when WP Slimstat is deactivated).
Also, WP Slimstat V2.6 browscap.php line 230 is causing errors in the server logs whenever it is called (i.e. on every page view)
"[30-Jan-2012 13:45:29] PHP Warning: touch() [function.touch]: Utime failed: Operation not permitted in /usr/home/../../wp-content/plugins/wp-slimstat/browscap.php on line 230"
PHP V5.2.12. This may be a file permission issue. (The install instructions for WP Slimstat have been followed).