Moore's law really help you with throughput, but latency still requires good engineering.

And you are right, that we got good UI latency even back in the 1980s. You just have make sure that you do the absolute minimum amount of work possible in the UI 'thread' and do gradual enhancement as more time passes.

As an example, the Geos word processor on the C64 does nice line breaks at the end of words only. But if you type really fast, it just wraps lines when you hit exactly x letters, and later when it has some time to catch up, it cleans up the line breaks.

That way it can give you a snappy user experience even on a comically underpowered system. But you can also see that the logic is much more complicated, than just implementing a single business logic for where line breaks should be. Complication means more bugs, more time spent writing and debugging and documenting etc.