Which option(s) is/are slowing down DHCPv4?

In /etc/config/dhcp, I did not have the following two options:

config dhcp 'lan'
 option dhcpv4 'server'
 option ra_slaac '1'

but do have:

config odhcpd 'odhcpd'
        option maindhcp '0'
        option leasefile '/tmp/hosts/odhcpd'
        option leasetrigger '/usr/sbin/odhcpd-update'
        option loglevel '3'

After adding those two lines under "config dhcp 'lan'", I notice that my DHCP lease retrieal or renewal takes much longer. The IPv6 SLAAC assignment arrives immediately, then a few (5?) seconds later, the IPv4 DHCP assigned address finally arrives.

Commenting out both of these two lines make the IPv4 DHCP assignment faster, but I'm wondering which one is more or solely responsible for slowdown. If it's the "option dhcpv4 'server'" line, why doesn't the "config dhcp 'lan'" section need this dhcpv4 option? Is it because of the later "config odhcpd 'odhcpd'" section that's handling dhcpv4 in place of dnsmasq?

odhcpd != dnsmasq

If I understand you correctly, then you may want to ensure you do not have another DHCP (v4) server on your LAN. You shouldn't get an IPv4 assignment if you turn this off.

Again, odhcpd (which does DNCHPv6 and RA) and dnsmasq (which does DNS and DHCP-v4) are not the same; and in fact, as you see, the section you quoted is not a LAN config (it's a Global config).