Fifteen seconds is the sprint format: all burst, no pacing. It's the shortest standard timer, popularized by minimal typing tests, and it answers one question — how fast can your fingers go when fatigue isn't a factor? Expect your highest WPM numbers here, and don't compare them directly with longer tests.
The real value of the 15-second test is as a training tool. It's the ideal warm-up (three quick rounds before a serious attempt), the fastest way to test a technique change, and the most addictive format for personal-best chasing — you can fit another attempt into any spare moment.
Eletypes runs it free with no account: live WPM as you type, accuracy and consistency stats at the end, history charted locally over time. When you finally crack your record, the shareable result card is one click away.