Slow performance when changing appearance settings with Cyrillic fonts

[RomanSoviak]

oryel_1984.zip
Hi! Thanks for the great SDK. I tried to open this book, but working with that has bad performance. For example on Windows, iOS this book opening fine. I don't know where the issue is.

[RomanSoviak]

UPD I put .epub file on drive https://drive.google.com/file/d/1erwVdLcv9-WJaYCgUBJTUb5CMK6yWcJz/view?usp=sharing

[mickael-menu]

I didn't notice any significant performance issue on a Pixel 3a. Sometimes a chapter takes too long (~2 s) to initially render – maybe because of the Cyrillic font? – but no issues turning pages.

However I noticed that the columns get shifted when swiping:

[RomanSoviak]

[RomanSoviak]

@mickael-menu thanks for the quick response. Sometimes I have this issue.

[RomanSoviak]

@mickael-menu It works slowly when I change settings(Size of font, ...)

[RomanSoviak]

@mickael-menu video from Xiaomi Redmi Note 5https://drive.google.com/file/d/1DOEHY_4Q1mO3ltEt5GPXiAc-uWXUCT5E/view?usp=sharing

[mickael-menu]

Thanks for taking the time to make a video. I can reproduce the issue too when changing the font sizes.

Like you mentioned in this other issue: readium/r2-testapp-kotlin#334, I think it's related to the Cyrillic fonts. Unfortunately, I'm not sure we can improve much in Readium, because we're using the native WebView for the rendering on Android.

@JayPanoz Do you think it's something that could potentially be improved in Readium CSS or no luck? Maybe we could have a lighter but less reliable way of changing the font size for such cases?

[JayPanoz]

That’s a good question, I’m assuming we may potentially encounter similar issues with CJK – I know some other apps take ages e.g. 10 seconds for Japanese in vertical writing for instance.

My gut feeling tells me to check HarfBuzz issues to possibly find some leads (this is the lib Android/Chromium is using for text shaping) but it might be anything on top of it. Also reminds me of @danielweck’s performance issues a few months ago in Electron – massive area to paint – so that could be another lead.

[mickael-menu]

I'm closing this because it is not clear we can fix this issue in Readium. Feel free to continue the discussion if you have some insights in how we could address it.