Since Swift Performance introduced in 2017 many other caching plugins implemented it. Usually you can achieve the best results with delaying scripts, however sometimes it needs some additional critical CSS, to show everything fine in the above the fold content.

If you delay all scripts, on some pages (especially if there are animations, or javascript rendered elements), without manually added extra critical CSS, the CLS can be increased dramatically in the field data, also it can affect the real user experience.

With Smart JS Delivery you can optimize the javascript execution without any custom coding. You can simply enable Async Execute and choose Smart Delivery. If Smart Delivery is enabled, Swift Performance won’t wait for the first user interaction to run javascript, but will load the scripts in the right order with automated timing.

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