When a server is slow and the dashboard is vague, a dozen standard commands narrow the problem to CPU, memory, disk, network or a single process in about five minutes. This is the order we run them in and how we read each output.
Our nightly report export ran perfectly when we typed it, and failed every night as a systemd service with node: command not found. Services get no login shell, no .bashrc and a working directory of /. Here is how we made the unit self-contained.
A queue worker with Restart=always was found dead in the morning with result start-limit-hit. systemd had stopped restarting it after five crashes in ten seconds while Redis was briefly down. How to read the failure, tune StartLimit and RestartSec, and stop flapping services from staying dead.