Last command's wall time in the Bash prompt?

.bashrc

# Prompt timer using only Bash internals (no traps or external commands).
# Displays how long the last command took if it was 5 seconds or more.

__timer_stop() {
  if [ -z "${__timer_start_sec:-}" ]; then
    # If `__timer_start_sec` was already cleared, clear `__timer_show` as well, so we don't
    # keep showing an outdated timer.
    unset __timer_show
    return
  fi
  __timer_show="$((SECONDS - __timer_start_sec))"
  unset __timer_start_sec
}

# This is run in a subshell (via PS1), so we cannot unset global vars here.
__timer_echo() {
  if [ "${__timer_show:-0}" -ge 5 ]; then
    echo "[last: ${__timer_show}s] "
  fi
}

# Trick to start the timer at each prompt without using traps or changing PS0 appearance.
# We exploit parameter expansion and $USER (any existing variable works).
PS0='${USER:${__timer_start_sec:=$SECONDS}:0}'

# Run stop function before each prompt.
PROMPT_COMMAND="__timer_stop"

# Show timer in prompt if applicable.
PS1='$(__timer_echo;)'"[\u@\h \W]\$ "

Will show prompt like

[last: 11s] user@machine ~$