A thicker tunnel barrier stops adding measured delay. What was likely timed?

Answer: Stored wave leakage

Particle crossing speed — Particle crossing speed is the tempting story, and it is why the Hartman effect sounded like faster-than-light tunnelling. The better reading is that the measured group delay is the wrong denominator for a traversal speed. Once it is treated as storage or lifetime, the paradox loses much of its force.

Stored wave leakage ✓ — Correct. For thick barriers, a measured group delay can saturate because it behaves more like the lifetime of stored wave energy or probability density than a crossing time. Think leaky cavity, not car tunnel. Making the barrier longer need not make the leak-out time grow linearly.

Detector cable delay — Cable delay is a real engineering nuisance, so it is a plausible first suspicion. But the Hartman-style saturation is a theoretical and experimental wave effect, not just sloppy electronics. The oddity remains even when the measurement reference is handled carefully.