Greenlet is a highly performant theme by default. However, it also provides the option to change the performance settings based on the user preference.
Access performance settings
To access performance settings navigate to Appearance > Customize > Performance
Defer CSS option allows the styles to load after the page load. This enables the browsers to render the content first and then load the styles so that rendering is not blocked when parsing the styles. Please note that enabling this option will let the browser to render the content without any styles first (hence users can notice the same) and then apply styles once the stylesheet is loaded.