I think that fly.io is still using firecracker for their microvm offering, although qemu has a similar machine type implemented.

Having worked on firecracker and with firecracker, it does a good job in the networking area.

Note that there's no multiqueue implementation available, so depending on how many connections and data you're passing through, it may start struggling if the workload is high.

I don't know the specifics of how DO sets up their VMs, but I assume that they're not doing any fancy networking optimizations, so you may end up with similar issues on both setups.