This is not actually a PHP question, but............
if you install the speed test extension for Chrome or Firefox, this will indicate where you can improve page speed.
Use LESS or SASS to create leaner CSS - but use the minified css files in your live site.
Minify your js files.
You can do the following;
Ensure that your aren't looping unnecessarily, e.g. using separate SQL queries, where a simple join could suffice.
Image quality should be optimised for web and if possible resized.
Once you have the site online, you may want to use this site to test the performance of your page. I have found it to be very helpful. In the results, the report indicates areas that need attention and provides a summary of recommendations.