I've been preparing a blog post about first input delay and did one more test on my Alacatel One phone and see that we have a really long task that happens late when testing https://en.m.wikipedia.org/wiki/Barack_Obama (after onload, the user see all content) that on my phone lasts for 881 ms = the user can't do anything = if the user presses a key etc it causes a first input delay. My guess is that this is the biggest cause of high FID on mobile.
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I've collected a trace log (you can drag and drop the trace-1.json file into devtools/performance tab in Chrome to see what it looks like) so you can use it to zoom in and better see where the time is spent.
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This point directly to ResourceLoader adding too much CSS at once. It seems like the client-side code is already chunking up CSS for @import calls that need to be at the top, so there is already some logic that could be reused to reduce the size of nextCssBuffer.