Why they're fast at different fires — and why the "which one is better" question is the wrong question.
Both technologies detect the same thing — smoke — but they sense it through completely different physical mechanisms, and those mechanisms happen to respond fastest to different classes of fire. An ionization detector watches a tiny electrical current for disruption. A photoelectricdetector watches a beam of light for scattering. Neither one "sees smoke" the way the other does, and that difference has real consequences for how quickly each one sounds an alarm depending on what kind of fire is actually burning.
The diagram below shows what's physically happening inside each chamber. The chart after it shows why that mechanical difference translates into a real gap in response time — and why modern detectors increasingly just include both.