OK performance is nice to have but frankly I think that's just managers shoving resources at that because they know how to measure it and a 10.3% increase in speed or memory efficiency or something is easy to sell in a meeting.
UI innovation is much harder to quantify, requires more work to sell, and the payoff takes much longer to manifest, not least because every change will generate a certain number of complaints from people who think their pet issue should have been addressed first, like UI designers are interchangeable with performance specialists.
Chrome shot to first place because it kicked Firefox's ass on performance and IE's ass on reliable rendering (an opinion which you may or may not agree with). But while extensions are wonderful they can't and don't replace core UI innovation. And tbh I'm really bored with Chrome at this point; from a user point of view nothing significantly new has happened for a really long time.
Look at the bookmark management, for instance. It's been the same since forever, and if I want to socialize that information or turn it into a feed or something I have to go looking for third-party solutions, which are thin on the ground because people are reluctant to develop too much for a platform over which they have little or no control.