Improve parsing speed for large WebVTT files

[xinyang123123]

After selecting the specified subtitle, it will appear 5-10 seconds later, but the audio is still synchronized after the appearance. I use TrackSelectionView to switch the subtitle. The version number is 2.8.4.

I am testing on Fire TV.

[xinyang123123]

When my vtt subtitle file is about 4M, it takes 30 seconds to load.

[ojw28]

Please provide complete information as requested in the issue template. The issue template can be found here.

[xinyang123123]

Issue description

When i try to load a subtitles which size over than 4M, the subtitles will show untill fully loaded. before loading successfully, there will no any signal shows subtitles is loading.
I tried in FireTV stick and FireTV ,there are 15-40 seconds loading time, and the subtitles shows correctly when loaded successfully.

Reproduction steps

Open the video and select subtitle.

Link to test content

Email.

Version of ExoPlayer being used

2.8.4

Device(s) and version(s) of Android being used

Fire Tv(7.1.2)
Fire Tv stick(5.1)

A full bug report captured from the device

No error.

[ojw28]

It takes about 3.5 seconds on a Galaxy S8. It looks like this is just how long it takes for our current implementation to parse the VTT file, which is pretty huge (over 127 thousand lines). If it needs to be downloaded as well, that will add additional delay.

I don't think we have any plans to process such files incrementally. It is probably possible to make the parsing code in WebvttCueParser significantly faster, since it looks like it's not particularly optimized currently. In particular, the use of Pattern is likely to be expensive, and there are a lot of new String instances being created during parsing.

Marking as a low priority enhancement. We would accept a high quality pull request that improves the speed of the parsing code.