I'm not really sure where things went wrong, but the GET requests for WHMCS clientarea pages since the v6.2 upgrade are legitimately about 6 times worse than that of the 5.X version.
Looking at the WHMCS demo, even a simple login page of 2.7KB has a GET request of a whopping 1.11s (see attached).
The login page in my 5.3 install has a GET request of only 200ms.
I've tried contacting WHMCS support about the issue, naturally they tried blaming my server, theme and action hooks, but all of that falls apart when their very own demo suffers from the same issue.
I've even seen a post in these forums regarding this exact same issue, which upon revisiting now results in a 404. It's clear to me WHMCS is well aware of the issue, but has zero ambition to fix it.
I'm writing this to hopefully get some awareness around the issue so we can get some sort of a fix, until then WHMCS seems content banking on the fact likely very few people will notice the issue.