This does not really have anything to do with either kubernetes or networks. If your computer is busy, it won't be able to process packets. Accessing certain kernel stats via proc, sys, or other special files may be really expensive. For example /proc/pid/smaps of a running mysqld takes 2 seconds on a computer I happen to have on hand. Sometimes when you have many cores it is expensive to produce some of the fields of /proc/pid/stat because the kernel has to visit numerous per-cpu data structures. /proc/pid/statm is better for this reason, if it contains what you are looking for.

TL;DR reading kernel stats can take a long time and cost a lot of CPU cycles. It costs more for more containers, and more on bigger machines.