How might software development have unfolded if CPU speeds were 20x slower?

Same as now. Driven by incompetent management.

This would be modern version of Steampunk :)

Performance is a feature just like any other. Customers are looking for a good combination of features, including performance. At some point, the feature (in this case performance) is good enough, and it doesn't make economical sense to improve it.

Microsoft specifically cared about all that, way more than 20 years ago - Windows, for all the criticism it takes, from early days had internationalisation (they wanted to sell across the world after all), accessibility features, ways to adapt to different screen and DPI sizes etc. I think Explorer windows since at least Win 98 did something responsive-ish in that various side bars/panels would disappear if you made the window narrow enough. Of course, the icons would rearrange themselves into fewer or more columns as you resized the window too.

Depends what you mean by "similar to VB6/Delphi".

The VB6/Delphi era (called at the time RAD for Rapid Application Development) was oriented around proprietary UI and component frameworks. They were planned out a long way in advance and solved a lot of problems up front, but, they were tied to Windows and then you had to buy the dev tools on top.

These days people usually want to write web apps if they aren't doing mobile. The web as a UI framework is basically evolved rather than designed. The features you need to support RAD don't exist. Moreover the web UI community would view them as bad practice!

RAD tools were typified by a few things:

1. A standard way to define components that is statically reflectable (so you can populate property and event editors), and get documentation on the fly, etc.

2. MVC: a standard way to separate code from layout.

3. Layout management that's integrated with the component hierarchy.

4. A standard toolbox of high quality, high effort components.

5. A marketplace for components (which means they need to be distributable in binary form).

The web has none of this and isn't going to any time soon. For example, there's no agreement on how components work. React has one or more different solutions, Web Components is a different one, Vue yet another. MVC has been abandoned: React doesn't know anything about that. Code and layout are intermingled together. JavaScript is dynamically typed, largely reflection-free language. You can't automatically work out what the props are for a React component short of parsing the source code. You can't easily distribute proprietary components either because there's no way to enforce licensing (so a lot of users wouldn't pay, in reality).

And then there's the components. Browsers don't provide good enough components out of the box, unlike Windows-oriented RAD tools, so devs have to invent their own. Then React makes things worse by conflating theming with the component library. The result is that every web app has its own unique set of widgets with its own unique set of limitations and bugs, very few of which ever reach a high quality level. Widget development is expensive, which is why RAD tooling centralized it and supported a marketplace, so devs could amortize out the cost of writing widgets across many projects. The whole React/Jetpack Compose approach basically spurns all of that and says if you want to change the way your app looks you get to redevelop the widgets from scratch. And everyone does want that, so there are floods of duplicated work that's open sourced and then abandoned, but nothing ever gets critical mass.