While such code is not always easy to read, I have to agree with Philip.
> I know that I tend to write big functions instead of breaking them down,
> but then my code tends to run fast. I feel there is some connection
> between these facts.
For maintainability, the worst program I ever worked on had just a single
function - main() - but it ran like greased lightning which was the main
requirement! Formatting it nicely with 'proper' functions reduced its speed
by almost 15%.