Startup time (video startup time, or time to first frame) measures the delay between a viewer pressing play and the first frame rendering. It is one of the most consequential QoE metrics: studies repeatedly show that every extra second of startup increases the chance the viewer abandons before watching.
It is the sum of many steps - entitlement and license fetch, manifest download, the first segments, and the initial buffer fill - so reducing it touches the whole stack: faster license/manifest paths, a conservative initial rendition for a quick start, CDN proximity, and low-latency techniques. Startup time and rebuffering are the two QoE numbers most directly tied to engagement and, for paid services, churn.
