I worked at a large service provider and had to fix bugs and improve performance in TCP/IP and Intel Ethernet drivers. I ended up really enjoying it so it became a hobby and consulting gig as well.
Awesome, you are kind of describing my dream job. Any particular hobby projects you are willing to share? I'm working on something myself trying to use eBPF/XDP offloading for a particular protocol
Lately I have been enjoying NetBSD on a EdgeRouter-4 (Octeon MIPS). I am trying to port some of the OpenBSD Octeon improvements like the MMC driver and maybe SMP support. On FreeBSD I have been pushing some changes to the 1, 2.5, and 10gbit Intel drivers that will decrease driver overhead a few percent.