Twenty years ago, my IDE was two terminal windows running VI, vertically tiled. On the left, I open header files, and on the right, source files (I was a C++ programmer). When I wanted to look up method signatures or member names, I would quickly look it up from the relevant header file. This could take anywhere between 5-60 seconds.

If you remind me that IDE features could improve this by a factor of 10x, you're absolutely right. But if we apply Amdhal's law to developers, their biggest time waste is often not the physical act of coding, but fixing logical and structural issues in their programs. And the best way to prevent such high level issues is for the programmer to have a good mental model of the code base.

So, the possession of a good mental model of your code base obviates the necessity (but not the convenience) of IDE features such as autocomplete/intellisense. But at the same time, reliance on such features hampers the formation of a good mental model.

My current compromise is to rely more on the "Go to definition" function than auto-complete.