Two many MenuItems causes rendering the page partially

[Versed]

When I add to my p:menubar more than around 50 MenuItems, I have odd behaviour: when I navigate to another page, the webpage will load a small part, the browser is rendering this small part (e.g. the first submenu-item) and wait till background work is done for then rendering the rest of the page completely.

Primefaces: 5.2.3, 5.3, 6.1
JSF: jsf-impl-2.1.28.redhat-8
JBoss AS: EAP 6.4.12, EAP 6.4.16
Browser: Chrome, IE, FF

If I shall add more information, then I will do this.