I'm very sorry for resurrecting this thread - but this thread is mostly the first hit you find if you google for something like "pfsense slow login" or "slow dashboard" and for me it initially did not help, checking my DNS-settings.
after hours looking for a solution I stumbled over this reddit post - https://www.reddit.com/r/PFSENSE/comments/lmubku/comment/gtj41yl/?utm_source=share&utm_medium=web2x&context=3
disabled "firewall logs" widget - and dashboard now loads fast and login is also fast. The strange thing is, that I have another machine with a virtual pfsense installed (pretty much same hardware - cpu, ram, disk) and had the dashboard equally configured (and mostly all other settings are equal too) where this long loading issue did not happen.