SourceOpenTuning

How much of a source libavformat may read before it has to answer what is in it — the setting that decides what "opening a file" costs over a network.

FFmpeg's defaults (5 MB probe, 5 s of analysis) are tuned for containers that hide their structure. The ones PrismCore takes describe themselves in their header: Matroska carries CodecPrivate, MP4 carries the whole moov. Measured against a 5.4 GB HEVC/EAC3 remux over HTTP, one open cost 1.31 MB across 3 requests at the defaults and 0.92 MB capped — and on a living-room Wi-Fi link to a media server that difference is the visible part of "Preparing…", paid twice because the routing probe and the remuxer each open the source.

The caps are a ceiling, not a target: libavformat stops as soon as it has what it needs, so a well-formed file reads far less than either number. What they bound is the pathological case — sparse audio, junk streams, a container that never settles — where the analysis would otherwise read megabytes to learn nothing new.