Run a few commands first before you disable everything. Maybe a few UTM engines are causing the performance issue, not necessarily the web gui or the firewall.
# get system performance status
# diagnose sys top-summary
if any of them running on high CPU/memory, than check the log;
# exe log filter category 1 <--- for Events. Without number for the full list.
# exe log display
At least you would know what is causing the performance issue, instead of just assuming that it' s the web UI.