Synopsis slow WHMCS. Nothing has changed. Inspect element shows nothing. Only the front-end, and nothing on the back-end.
The only clue we have is the timeouts are long, eventually seeing fixed at around 60 seconds.
We start by watching the MySQL:
watch -n1 'mysql -uroot -e "show processlist"'
This proves inconclusive.
Next we watch for outgoing web connections:
watch -n 1 "lsof -i :443 -i :80"
This shows many Cloudflare attempts during clicks.
Next we look for resolved DNS:
sudo tcpdump -n -l -i any udp port 53 | grep -oP 'A\? \K\S+'
This is the final clue.
There is a module in WHMCS that’s connecting to Cloudflare and timing out. We can see the exact DNS name so we can find the module and disable it.