The most important part of the speed optimization is Critical CSS.
Swift Performance uniquely generates Critical CSS on the fly for all pages. You can choose Unused CSS method or Viewport Based (recommended).
Note: It can cause issues if your CSS is not valid. Unfortunately many plugins contains some CSS issues. If some of them breaks the Critical CSS, you should exclude them from merging.
To generate Viewport based Critical CSS our API will render the page, and generate CSS which is required for “above the fold” content. It is the better way, also it won’t break Critical CSS generation if the original CSS contains some smaller issues.
Extra Critical CSS