Using top on macOS
In the afternoon I noticed that my Mac mini 2014 running macOS Mojave was running slow. Even typing was sluggish. So I typed the following in an open terminal:
to find the culprit, which turned out to be Thunderbird, using 67% CPU.
top -u is an alias for
top -o cpu -O time
meaning it sorts first on the CPU column, and on the time column next.