You can measure performance in most browsers, however let's look at you can do it with chrome - note that firefox usage is pretty similar if you install the pagespeed addon.
- Open you page in chrome browser.
- Click menu view / developer / developer tools.
- Click on the Audit tab in the developer tools window.
- Click Run.
- Step through each of the audit report items in turn and address the significant ones you can.
- Resource caching - resources (images / CSS / JS) don't tend change much, so it's good to implement an appropriate cache policy by setting appropriate HTTP headers.
- Enable GZIP compression - most browsers, devices now support GZIP compression. Compressing content to be sent over HTTP, even with the overhead of compression and decompression on server and client side, so this is often valuable optimisation.
- Optimise images - large high resolutions images can be quite a significant part of the pay load. Sometime these can be compressed without loss, sometimes you may want to reduce the size for smaller screens.