OpnSense is incapable of proper DHCPv6-PD, that's when your route receives a prefix from upstream and delegates parts of it downstream. More specifically, it does the delegation, but it doesn’t add the relevant routes, effectively blackholing the allocated prefixes.
VyOS fixed this specific bug since I reported it. RouterOS and IOS never had it.
It really depends on the specific hardware. I have Mikrotik routerOS CHR that routes between VLANs at 6Gbit/s without breaking a sweat on a $300 intel box.
At the same time, some managed switches are dirt-cheap nowadays and they generally can push the traffic around as fast as it comes in.