Earlier quoted context omitted.
Almost always, nobody has any intention to "make it run buggy." It's about making it run well on platforms you care about (and officially support), and ignoring the others. Complicated software is usually fragile enough that it will have similar results.
If a user agent change makes the software slower for a platform then that's not exactly an accident so somebody must have had the intention to make it run buggy. This has nothing to do with 'making it run well on platforms you care about' but it is 'making it run bad on platforms you don't care about'. It takes more code for the software to act in this way (this is what proves this is intentional and not accidental),…
That's not necessarily true. For example they might be doing polyfills that are slower than the native windows implementation.
I feel like I'm seeing a lot of this kind of thinking everywhere. The canonical example for me is lately is "he voted for Trump/Hillary therefore he must support groping/war/whatever".
I wish people would ease up on these kinds of deduction-based motivation attribution. It's my view that there enough self-professed unethical motivations for us to work against, without speculating. The speculation just muddies the waters.