Summarize:

Website that use lots of Flash or Java can take much longer to load. Try not to rely on these for your basic web page, and don't make Flash videos play automatically. This is an issue with sites which automatically play embedded (e.g., YouTube) videos as well. Large, high-quality images can slow down your website considerably, and may burn through your server hosting space as well. Many photo editors and website hosts alike offer an option to optimize your photos for web publishing, so take advantage of this option.  Most website hosting services also offer an option to load images as viewers scroll down rather than loading a whole page's worth of images at once. While images are important when attempting to make your site more attractive, they can considerably slow down traffic from slower browsers and platforms. Don't overuse images. Minification removes all unnecessary characters from code, typically white space, new lines, and comments, which will both help your site load faster and boost your website's search engine optimization.  Closure Compiler by Google can minify Javascript for free, while other free minimizers can be applied to different types of code (e.g., HTML). You will still be able to edit your code with the human-readable organization. The code is minified only when it is uploaded to the server. Make sure you are using the latest version of PHP so you can benefit from the latest improvements. While the PHP developers try to keep the language backwards-compatible, you will probably need to spend some time updating your code in accordance with the latest changes. If you're using a website hosting service such as WordPress or Weebly, you shouldn't have to worry about manually updating your website's PHP. Enabling this option for your webpages' files, photos, and other content will ensure that users who allow cached content won't have to retrieve the content every single time they revisit your site. Any users who clear their browser's cache and cookies before revisiting your site won't benefit from this, but the bulk of people who come across your site will be positively impacted.
Reduce your use of intensive applications. Optimize your images. Minify CSS and Javascript code. Use the latest version of PHP. Make sure your website caches files locally.