Nobody likes slow websites. The challenge is to keep as many features as possible, while keeping the page load time low.
Optimize Images
Images account for much of the time spent downloading content for a web page. Optimized images are smaller in size, therefore download faster. This is especially helpful for mobile devices.
You can find free tools online. We prefer ImageOptim for Mac.
PHP 7
If you're still running PHP 5 you should upgrade to PHP 7.x if you're able. There is a noticeable performance increase. This is especially true for more complex sites.
BigPipe (Drupal 8+)
The BigPipe module is included with Drupal 8. It allows the browser to render the page in parts rather than waiting for the entire page to download first. If it's not enabled, it's worth a try.
Advanced CSS/JS Aggregation (Drupal 7, 8, 9)
AdvAgg is a Drupal module which compresses and aggregates CSS and JS. This reduces the number of downloads, and the size of each. You'll need to test the various settings as some settings can cause javascript code to break.
Lazy-Load (Drupal 7, 8)
The Lazy-Load module allows you to easy lazy load images used in fields and in content. This can make a big difference if you have a lot of images "below the fold."
