Unfortunately, GTmetrix does not analyze password protected sites.
I only use the Browser's dev tools. Your homepage completely loads not more than 6s. So that is kind of fast considering you have a number images on your homepage.
The flickered issue that you're seeing is when the down-chevron-arrow shows-up moves the menu items a bit. You can install a caching plugin to improve the loading of your site fonts and CSS.
Improving Site Performance Tips