It's not only filesize it's also what the browser must parse. Parsing huge files is not a great experience on mobile.
I'm trying to imagine what browser is so outdated that it struggles to parse 100kb of javascript
It's not the browser, it's the processor that's in most non-iPhones.
Samsung phones and Pixels aren’t that bad either.
I am not saying it to be proud, but I tested our 3mb internal app with a 5 year old android phone, and worked.
The only time it was slow is if the browser itself was old (e.g. Chrome 44).
All browser processes 100KB of scripts just fine. And they process the scripts in the background. The HTML is already parsed and displayed.
For low end android devices on a network in rural areas, that's a lot.
I would guess that for the majority of JS-heavy websites, android users in rural areas probably do not make up that much of the target market.
It's unrelated to network conditions (those just make it worse). It's related to the processor in many mobile phones.