We have noticed that the WordPress Customizer can get a bit laggy in the Google Chrome browser. If you are finding the Customizer “laggy” (slow) when opening and closing tabs and panels we suggest you try using the latest version of Firefox to see if things are faster. We have found Firefox to work much better while using the Customizer, of course results may vary depending on your browser addons and cache.
We have also noticed that if you have many tabs open in your browser it can slow things down, especially when you have videos such as Youtube or Netflix playing on another tab. We recommend working in the Customizer while all other browser tabs are closed to ensure it’s as speedy as possible.