I don't know if it's because I'm becoming older (in developer age). One of the hardest things I have to do is dismiss front end performance. I came of age in a web where everyone had a crappy device that could barely run JavaScript. Add too many lines of code and the device crashed. Add several libraries and the page visibly crawled as you scrolled. Load that extra script and now you have excluded all IE8 users.
After each deployment at work, I opened and tested the pages on IE tester. An application that could run several versions of Internet Explorer. Then I'd open the pages on my HTC Evo mobile phone. Other devs will test on their own respective device. We made sure it worked everywhere. It was a pain.
Today, I'm still afraid to deploy JavaScript code because it might fail in an unexpected manner on some device. On my brand new phone, some pages still seem like they are loading with difficulty. Especially those built with React. But one thing has changed in the way we deploy our code. Almost nobody cares about how our code performs on the client side.
In fact, I worked with several companies that have no way of knowing how their website is performing. They don't check which browser their customer uses, and they don't track performance in any way.
Is this it? Did devices evolve in a way that we don't need to check on them anymore? Maybe so, but I'm still skeptical.