This is neat. I've been doing the same with Pi 1s and 2s for a few years (http://github.com/rcarmo/raspi-cluster) but stopped short of upgrading to 3s and 4s because of heat/power/cost.
I've found that it is possible to run k3s on SD cards quite well if you're patient, but that the Pi 2 master node spent a _lot_ of time just doing internal stuff even on an otherwise idle cluster, so I went back to Swarm.
(I also had quite a bit of trouble with setting up a container registry, which was hard and undocumented in the first few releases of k3s. Ingress was also a bit of a pain, it's much easier to get traefik working in Swarm...)
Ok so it's just the API servers. Not every node which is what it sounded like. Typically you wouldn't run normal workloads on the same host as the API. Although these quick "how I did it" articles have a tendency to skip best practice. Still sounds like something isn't right in your environment. They shouldn't be hammered that hard unless you have a very large cluster.
I've found that it is possible to run k3s on SD cards quite well if you're patient, but that the Pi 2 master node spent a _lot_ of time just doing internal stuff even on an otherwise idle cluster, so I went back to Swarm.
(I also had quite a bit of trouble with setting up a container registry, which was hard and undocumented in the first few releases of k3s. Ingress was also a bit of a pain, it's much easier to get traefik working in Swarm...)