Quick test done on a VM with 1 CPU and 1G RAM:
vyos@1.4.0-rc1# for I in {1..2542}; do set firewall ipv6 name Test rule $I action accept ; set firewall ipv6 name Test rule $I destination port $I; set firewall ipv6 name Test rule $I protocol tcp ; done
vyos@1.4.0-rc1# time commit