Bugzilla

In [bug 1974237 comment 2](https://bugzilla.mozilla.org/show_bug.cgi?id=1974237#c2), running following script on this [Dash player](https://reference.dashif.org/dash.js/nightly/samples/dash-if-reference-player/index.html?mpd=https%3A%2F%2Fed14.cdn.svt.se%2Fd0%2Fworld%2F20250623%2F9ba84fdc-325a-426b-896f-6d0a14a29226%2Fdash-full.mpd&autoLoad=true&muted=true+&debug.logLevel=5&streaming.capabilities.supportedEssentialProperties.0.schemeIdUri=urn%3Advb%3Adash%3Afontdownload%3A2014&streaming.capabilities.supportedEssentialProperties.1.schemeIdUri=urn%3Ampeg%3AmpegB%3Acicp%3AColourPrimaries&streaming.capabilities.supportedEssentialProperties.2.schemeIdUri=urn%3Ampeg%3Adash%3Aurlparam%3A2014&streaming.capabilities.supportedEssentialProperties.3.schemeIdUri=urn%3Ampeg%3Adash%3Aurlparam%3A2016&streaming.capabilities.supportedEssentialProperties.4.schemeIdUri=urn%3Ampeg%3AmpegB%3Acicp%3AMatrixCoefficients&streaming.capabilities.supportedEssentialProperties.5.schemeIdUri=urn%3Ampeg%3AmpegB%3Acicp%3ATransferCharacteristics&streaming.capabilities.supportedEssentialProperties.6.schemeIdUri=http%3A%2F%2Fdashif.org%2Fthumbnail_tile&streaming.capabilities.supportedEssentialProperties.7.schemeIdUri=http%3A%2F%2Fdashif.org%2Fguidelines%2Fthumbnail_tile&streaming.delay.liveDelayFragmentCount=NaN&streaming.delay.liveDelay=NaN&streaming.buffer.initialBufferLevel=NaN&streaming.liveCatchup.maxDrift=NaN&streaming.liveCatchup.playbackRate.min=NaN&streaming.liveCatchup.playbackRate.max=NaN) (VPN needed, not working for US)

```
(()=>{
  const track = $('video')[0].textTracks[0]
  const newMode = track.mode === 'showing' ? 'hidden' : 'showing'
  const before = performance.now()
  track.mode = newMode
  const elapsedTime = performance.now() - before
  const cues = track.cues?.length ?? 0;

  console.log(`Setting text track mode to ${newMode} took ${elapsedTime}ms (${cues} cues)`);
})()
```
The result BEFORE optimization on own debug build is 
> Setting text track mode to showing took 69586ms (38780 cues) (original)

The result AFTER optimization on own debug build is 
> Setting text track mode to hidden took 175ms (38780 cues)

That shows the improvement around **397.63x** faster. I will update my patches later after finishing testing.

In [bug 1974237 comment 2](https://bugzilla.mozilla.org/show_bug.cgi?id=1974237#c2), running following script on this [Dash player](https://reference.dashif.org/dash.js/nightly/samples/dash-if-reference-player/index.html?mpd=https%3A%2F%2Fed14.cdn.svt.se%2Fd0%2Fworld%2F20250623%2F9ba84fdc-325a-426b-896f-6d0a14a29226%2Fdash-full.mpd&autoLoad=true&muted=true+&debug.logLevel=5&streaming.capabilities.supportedEssentialProperties.0.schemeIdUri=urn%3Advb%3Adash%3Afontdownload%3A2014&streaming.capabilities.supportedEssentialProperties.1.schemeIdUri=urn%3Ampeg%3AmpegB%3Acicp%3AColourPrimaries&streaming.capabilities.supportedEssentialProperties.2.schemeIdUri=urn%3Ampeg%3Adash%3Aurlparam%3A2014&streaming.capabilities.supportedEssentialProperties.3.schemeIdUri=urn%3Ampeg%3Adash%3Aurlparam%3A2016&streaming.capabilities.supportedEssentialProperties.4.schemeIdUri=urn%3Ampeg%3AmpegB%3Acicp%3AMatrixCoefficients&streaming.capabilities.supportedEssentialProperties.5.schemeIdUri=urn%3Ampeg%3AmpegB%3Acicp%3ATransferCharacteristics&streaming.capabilities.supportedEssentialProperties.6.schemeIdUri=http%3A%2F%2Fdashif.org%2Fthumbnail_tile&streaming.capabilities.supportedEssentialProperties.7.schemeIdUri=http%3A%2F%2Fdashif.org%2Fguidelines%2Fthumbnail_tile&streaming.delay.liveDelayFragmentCount=NaN&streaming.delay.liveDelay=NaN&streaming.buffer.initialBufferLevel=NaN&streaming.liveCatchup.maxDrift=NaN&streaming.liveCatchup.playbackRate.min=NaN&streaming.liveCatchup.playbackRate.max=NaN) (VPN needed, not working for US)

```
(()=>{
  const track = $('video')[0].textTracks[0]
  const newMode = track.mode === 'showing' ? 'hidden' : 'showing'
  const before = performance.now()
  track.mode = newMode
  const elapsedTime = performance.now() - before
  const cues = track.cues?.length ?? 0;

  console.log(`Setting text track mode to ${newMode} took ${elapsedTime}ms (${cues} cues)`);
})()
```
The result BEFORE optimization on my debug build is 
> Setting text track mode to showing took 69586ms (38780 cues) (original)

The result AFTER optimization on my debug build is 
> Setting text track mode to hidden took 175ms (38780 cues)

That shows the improvement around **397.63x** faster. I will update my patches later after finishing testing.