The Floating Timer app features both a countdown timer and stopwatch that will float on top of other running apps. This app is useful to time activities such as: Exam practice, gaming speed runs (speed-running), gaming boss fights, cooking.

Usage:

Drag to move timer position
Tap to start / pause
Double tap to reset
Drag to trash to exit

Premium version unlocks:

Simultaneously run more than 2 timers (multiple timers)
Change Timer size and color

Open source: https://github.com/tberghuis/FloatingCountdownTimer