Changing the Overview graph will change how many days to draw, it
wouldn't affect the plugin performance.
I recommend deactivate the plugin(s) and test the site speed before and after. Check the performance using P3 (Plugin Performance Profiler) http://wordpress.org/extend/plugins/p3-profiler/
On Statcomm overview, below all the boxes, there is a 'Time Summary' box where you can see render time and table size, among other information. Typical times should be under 5 seconds.
If you want to reduce the database size I would recommend change settings 'Automatically delete visits older than' to 3 months for example and 'Automatically delete spiders older than' to one month or suppress it completely using 'Do not collect spiders visits' checkbox.
Disabling User Agent String Database will disable also the ability of Statcomm for detecting OS and browsers. This should be used in special cases when you are certain you have plugin problems not necessarily performance ones.
Currently I'm testing the plugin with 500MB tables and there is no performance hit.
Hope this help you!