today I had issue with WHMCS loading very slow. I've checked for denial of service attach, MySQL database queries report and then I realised that this looks like a timeout. I had to use Wireshark to find out what is going on. Issue was with accessing license server to one of WHMCS addons which I have installed. Can WHMCS licensing system can be upgraded to have some failsafe time where license can not access addon license server but not cause timeouts for whole WHMCS installation?
EDIT: There is one more thing. IMHO next to each addon there should be some icon telling that it has accessed licensing server correctly. It took me some time to find which addon is causing timeouts. If there would be some icons for that then it would be a lot easier to react on similar situations in future.