Performance is important, but developer productivity matters just as much. I'm curious how this balances both.

As should be clear from the screenshots, in the 80's and 90's we were entirely unable to perform without gradients everywhere. Even my school notebooks where full of pencil- and pen-drawn logos full of gradients. At least one submitted essay had a heavily gradient-laden front page.

More seriously, most of the scene were teenage boys who were playing, not doing serious work. While some of the tools were mostly productivity focused (e.g. the assemblers, trackers), perceived cool-factor outweighed productivity for a huge proportion of them.

For the trackers in particular, while there were still the embossed metallic look and sometimes gradients, the dense UI is largely a factor of small screens (often down to 14" monitors) and very low resolutions giving you very little to play with. And the thing is, while it's messy, everything is there on one screen, and almost every tracker followed almost exactly the same principles and layout, so you learnt quickly enough.

Productivity was usually excellent once you discovered and learned all the little details of the UI, which could take a while (think of vim versus regular text editors). Most of those tools were basically 'single-page-apps', e.g. all features crammed into a single UI page, but it also meant that all features were only one mouse click away.

Yes but going really far down that path has given us sterilized UIs that have no character and no personality. Every website and app looks more or less the same. I, for one, enjoy a thriving baroque culture of weird interfaces with different levels of usability. Given the nature of the people who write these (usually for themselves or their teams), I'm sure usability was important and good atleast for their own audiences.