Search online for "software optimisation techniques" and you will get lot of tutorials on how to optimise software such as data structure packing, locality, cache optimization, loop unrolling etc.

For self-study, as others have mentioned, keep the manuals of CPU as well as network devices you are using handy. It will help you understand the low level code in the kernel as well as the related drivers.

For getting started with Linux kernel development, start at these places:

https://kernelnewbies.org

https://www.linuxfromscratch.org

https://lwn.net/Kernel/LDD3 (outdated but still helps build your frame of mind navigating the device driver code, in your case network drivers)

https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/lin...

https://beej.us/guide/bgnet

https://man7.org/training/download/Linux_System_Programming_...

https://igm.univ-mlv.fr/~yahya/progsys/linux.pdf

https://users.cs.cf.ac.uk/Dave.Marshall/C

https://www.dpdk.org

For some fun way of understanding about computer architecture in general, have a look at:

https://www.coursera.org/learn/build-a-computer

https://www.coursera.org/learn/nand2tetris2