What is Wait Time on Pingdom

Hi there,

I'm hosting Wordpress on Linux server on SSD, 32 Gigs of ram, a lot of free ram, a lot of free CPU resources
But the customer complains about the speed. (not a bandwidth issue)
My server use the Plesk 17.8.11 Apache PHP 7.2.x nGinx as front end accelerator

Screenshot

Where should I look ?
I don't think it's WP related as it seems to wait before the wp starts to load some stuff'

Thanks to help me

Regards