rmon

rmon is a lightweight system monitor that lives in the Ubuntu top bar.
It shows a compact, always-visible readout of overall CPU load, memory
usage and swap usage, updated every couple of seconds.

Click the indicator for full detail: usage per CPU core, memory and
swap in human-readable units, and a few preferences (refresh
interval, which metrics show in the top bar label, whether per-core
detail is shown in the dropdown).

rmon reads /proc/stat and /proc/meminfo directly -- no telemetry, no
network access, nothing phoning home.