The Basic Rules Of Speeding Up WordPress load-times

Well basic rules are as follows:

1.) Don’t use too many plugins. A range of 20-30 max plugins installed and activated should be the “rule of thumb”. Using plugins impacts your server negatively in terms us each plugin use more server memory, and adding load times in terms of individual css, js and php files etc. which must be loaded and read each time a browser client request a page on your website. Add on top of that the increased need for read request to the database too.

Use real code instead if possible. It is always faster and will have much less impact on your system/server.

2.) If you must use a plugin use a MU-Plugin and also load code into that instead of functions.php. Just note that it will impact the whole network if you use WPMU, (E.g. all sub-sites and the primary site):
WordPress mu-Plugins: Your Guide to Using Must-Use Plugins

3.) Use server side caching like WP-Rocket or Hummingbird

4.) Use Browser client caching

5.) Use a CDN nextwork such as CloudFlare or Max CDN

6.) Combine CSS and JS files if possible:
  ** How to Combine External CSS in WordPress
  **How to combine CSS and JS files for WordPress

7.) Make sure your server have enough memory allocated to your WP site. Depending on if you run a Stand-alone WP Install or a WPMU network site is huge difference also in memory requirements.

8.) Make sure you have Gzip compression enabled:
  ** Enable Apache Gzip Compression (mod_deflate) in cPanel Account
  ** Check Gzip (& Brotli) Compression

9.) Make sure that your website is optimized for mobile too!
  ** 8 Quick Tips to Optimize WordPress Website for Mobile Users

10.) Minimize the number of DNS queries/lookups needed when a client request and loads your site:
  ** How to Reduce DNS Lookups? (Tips to Fasten Up the Speed)

11.) Don’t use larger images than you have to and make sure they are optimized for web so they do not weigh more than they really have to. Smaller images reduces the load times drastically. Also use jpg, webp over png if you do not need transparency in your image. Also use a image optimization plugin like WPMU Dev Smush or Imagify for even faster delivery times.
  ** An Introduction to Image Formats for the Web in 2020
  ** Image Optimization For The Web: The Definitive Guide

I guess that is most of it; if I have missed something then maybe add a comment below in this forum thread?