Calculate time with the right assumptions
For an uninterrupted twelve-minute recording played at 1.5 times normal speed, the ideal viewing time is eight minutes. That calculation assumes the player maintains the selected rate and the viewer neither pauses nor seeks. A ten-minute real session is not automatically evidence that the speed setting failed.
Accelerated playback also consumes available media time faster. A buffer containing thirty seconds of programme material lasts about twenty wall-clock seconds at that rate, assuming no replenishment. Faster progress can therefore reduce the reserve against a slow transfer even though the network connection is unchanged.
Pitch and rate are separate controls
MDN documents pitch correction through preservesPitch. Changing playback rate and preserving perceived pitch are related but different operations. A supported player can try to keep voices natural while advancing the timeline faster; an implementation or control may expose different choices.
Do not infer a speech-language change from a pitch change. Likewise, voices sounding different at a speed setting does not establish that another audio track was selected. Compare the same content position at normal speed before changing output-device settings or blaming a damaged soundtrack.
Respect live and implementation limits
A fixed recording has later material available to consume, while a continuing live presentation has a moving edge. A player cannot accelerate indefinitely beyond media that has not been produced. Catching up to that edge and remaining at an accelerated rate are different tasks.
Rate support, supported ranges and reverse playback vary by implementation. Record the requested rate and the observed value rather than promising every browser accepts every number. If sound is muted at an extreme rate, compare ordinary speed first and retain the browser version; the symptom alone does not identify a connection or account fault.
Questions about this guide
Does 1.5× mean downloads are faster?
No. It changes the rate of media-time consumption.
Will every browser support reverse playback?
No. Do not assume negative-rate support from the presence of a speed control.