Step response

Apply a sudden change (raise the setpoint, switch on the heater) and record what the output does. The resulting curve tells you most of what you need to know about a loop: rise time (how quickly it gets near the new value, often measured from 10% to 90% of the change), overshoot (how far past the target it goes), settling time (how long until it stays within a stated band around the target), and steady-state error (any offset that never goes away).

A step test on the uncontrolled system is also the simplest form of system identification: the delay and time constant you observe give a first model for tuning a PID loop.

For founders, a step-response plot is a more honest performance claim than "fast and precise". It belongs in the test plan and the specification, with numbers and test conditions stated.