When I am in the customizer if i try to edit something like a slider I don’t see any changes in the live preview. However, if I untick a check box or something (which forces the page to reload), i see the changes i’ve made.
Its the same when I edit something in the additional CSS section. Any changes here won’t be seen unless the page reloads.
But its probably best to ask PerfMatters support. The developers use GP and are super proactive.