How should I set my Drupal web for better performance when I'm already satisfied with my theme and won't do many new theme adjustments?
The settings is probably in
If you don't anticipate important theme changes in the near future, you can well extend the cache expire time of your asset files like CSS and image files to multiple hours. You can do this via Expire rules in .htaccess.
For optimal efficiency, I'd recommend installing a Varnish proxy cache that accepts requests on port 80 and forwards them to Apache (running on another port, say 81). Varnish will then cache and deliver your asset files for multiple hours, thus reducing the load of the web server heavily. After installing and configuring Varnish and Apache, visit the performance page to also activate external caching of pages and set the page cache lifetime to a value you deem sensible.
We've seen huge improvements in page delivery time and server load by including Varnish in our Drupal hosting packages.
You can swap out the default drupal caching mechanism for a memcached Implementation, also you could try using a nginx instead of an apache.