Hello,
1. Yes, you can remove unnecessary elements if you don’t use them. To reduce the size of the DOM you need to reduce the number of elements on the concrete page.
2. Yes, if you untick the option then that CSS will not be loaded.
3. In general, our theme is already optimized and doesn’t have too many resources to make your website slow. Most likely, your website performance suffers from the content you have added to your website. It may happen in case of too many plugins, large not optimized images, or other resources. So we suggest you remove unnecessary plugins, install some kind of JS & CSS optimizing plugins like Better WordPress Minify and a caching plugin like WP Super Cache or use WP Rocket for both tasks.
Kind Regards
XTemos Studio