Another opinion →
I worked on that new Jira UI (codename Bento) and it was a constant source of embarrassment for me. I pushed hard to get the powers that be to focus more on performance and payload, but it was a Sisyphean task. It was perhaps the most demotivating job of my career because I disagreed with virtually every decision that was ever made with regards to that UI, from technology choices (the inane choice to use RxJS despite literally no valid use cases, the 5+ Redux stores in the same page, etc) to UI layout decisions (insane whitespace, the baffling choices about which things go in the right hand gutter…)
The components were mostly handed down to us from a company-wide UI team whose main design goals were to bring commonality across all Atlassian products. This is a worthwhile goal, no one thinks that it makes sense that Atlassian has different editors and even markup languages between (and even within) products. But the result was dog-slow overweight components like the Atlassian editor which tried to be everything to everyone. So when you need to bring in a monolithic highly-customisable 12mb+ component for every text edit field then things spiral out of control quickly. One cool thing, though, is that these components are open source. And it looks like some performance/payload work has been done since I left, thank god.