WHMCS "default" template font is named Ubuntu, so its a non standard font which is loaded from https://fonts.googleapis.com/css?family=Ubuntu:500,700 causing extra time to load a WHMCS site.

One can change this in /templates/default/css/whmcs.css (first backup file before editing)

On the top is line:
@import url("https://fonts.googleapis.com/css?family=Ubuntu:500,700");

just comment it out lke:
// @import url("https://fonts.googleapis.com/css?family=Ubuntu:500,700");

Then lookup for "Ubuntu" (i had 4 occurences similar to this:

font-family: 'Ubuntu', sans-serif;

so i changed it to:

font-family: sans-serif, 'Ubuntu';

so sans-serif is prefered.

The Sans-serif font looks very very similar almost same like Ubuntu one, but this way you decrease website load time..