I agree that bitbucket provides a nightmarish experience, but does it take in account that it's a Single Page App and thus after the initial loading the traffic will be much less?
Having used BitBucket since 2013, I've seen their performance ebb over the years. The most recent redesign made the biggest difference. I left some comments in their beta period noting that the performance was painful in areas, and it did improve some by the time it hit release, but was still noticeably slower than it had been before the redesign.
I would be interested in some sort of analysis of performance differences across web frameworks. Looking at BitBucket's code in the dev tools, they're using React + Redux currently. BackboneJS's web site lists BitBucket as using Backbone, so I would deduce the old, less slow design was the one that used Backbone. I've worked on both a snappy Backbone SPA and a lethargic AngularJS SPA, as well as in-between sites with Angular and React. But the business domains differ, so I don't have as apples-to-apples of a comparison as BitBucket's switch from Backbone to React would provide.
It would be an interesting experiment to get several teams of developers with similar levels of experience in different frameworks - Angular, React, Backbone, Vue, JQuery, all implementing sites to the same business specifications, and compare the relative performance (in both features delivered and page responsiveness) a year or two in. Practical to perform that experiment, probably not, but I'd certainly read the results.
So generally yes SPA are heavier, but they are more powerful. It's a matter of tradeoffs.
I don't think I've _ever_ seen one that's faster than the normal webapp it replaced. Except possibly for Google's first attempt on gmail, which was pretty snappy. Fortunately they've since replaced it with a sluggish behemoth, so order has been restored to the universe.