ARP slow to load
I've always noticed this with pfSense. Every router we have (hundreds) the Diagnostics -> ARP page is incredibly slow to load. If i run arp -na from the command line, it comes back almost instantly. Running arp -a takes a bit of time too, but maybe half the time the gui does. The gui page takes a minute or more to load, however.
The ARP table page attempts to correlate entries with DHCP leases and reverse DNS resolution for hostnames. Either one of those could account for a delay.