Emacs provides several functions and primitives that return time, both elapsed and processor time, used by the Emacs process.
This function returns a string representing the Emacs
uptime—the elapsed wall-clock time this instance of Emacs is
running. The string is formatted by format-seconds according
to the optional argument format. For the available format
descriptors, see format-seconds. If format
is nil or omitted, it defaults to "%Y, %D, %H, %M,
%z%S".
When called interactively, it prints the uptime in the echo area.
This function returns the processor run time used by Emacs, as a Lisp timestamp (see Time of Day).
Note that the time returned by this function excludes the time Emacs was not using the processor, and if the Emacs process has several threads, the returned value is the sum of the processor times used up by all Emacs threads.
If the system doesn’t provide a way to determine the processor run
time, get-internal-run-time returns the same time as
current-time.
This function returns the duration of the Emacs initialization (see Summary: Sequence of Actions at Startup) in seconds, as a string. When called interactively, it prints the duration in the echo area.