The demo runs on a GPU that powers down when nobody is using it, so the first visit after a quiet spell has to start it back up. Nothing is wrong. This page hands over on its own the moment it is ready.
Cadence, ground contact time, stride length, vertical oscillation and duty factor, from a single smartphone video.
Pose landmarks are tracked frame by frame, foot contacts come from ankle vertical velocity, and your declared height sets the scale.
A few seconds of a runner filmed side-on, plus the pace and the runner's height. No wearable, no lab hardware.