Customizer in the admin is very slow. Anytime I go into the Customizer it freezes for about 10 seconds then takes another few seconds to save after a change is made. This happens on a demo site with no plugins.
To check if it’s an issue with Sydney theme’s code, please try temporarily switching to one of default WordPress theme. And try the customizer. Does it still resulting the same?
Check your server and ensure it meets the minimum requirements as mentioned here: https://wordpress.org/about/requirements/