A mobile network is basically sub-Saharan. 120 KB of JavaScript is far too much to download, decompress and evaluate on a phone.
My Nexus4 has a faster internet connection and more RAM than my parent's laptop, and it's almost a 3 year old phone. Most (75%+) smartphone users in the UK have devices that can easily cope with a 120kb javascript library. Your rule is wrong for western countries.
As someone who frequently travels on the tube: The rule persists.
And many western users have a limited amount of data on their plans.
Use case also matters. If you're delivering 1mb of javascript for your ecommerce website, that's too much.
That said though, if your ember.js code blocks rendering, or causes lots of repainting, and you're building a site rather than an app, then I'd question if you're making the right choice.