Good question, it's bits and pieces. I know there's more we can do with the userspace stack - netstack has some support for GRO/GSO, but unless I'm forgetting a detail we haven't fully plumbed that yet. It would definitely be interesting to do so - avoiding TUN turnaround while still utilizing mmsg and so on should provide excellent performance for something like a tsnet/libtailscale based server. We did recently improve performance in that configuration by enabling SACK, which is very significant.