Depending on how you count, the ratio might not be that small. A lot of hot code are written in hand-coded inline assembly, so in terms of CPU cycles run it's probably non-negligible.
i.e. take a look at the glibc implementation of 'strcmp` [0]
> A lot of hot code are written in hand-coded inline assembly
I know... I write GPU assembly for a living... And still I make that wager. It's not a lot. It's not even a little. It's an epsilon (overall). And it gets smaller over time.
What do you think the percentage of computation is with respect to hand-written vs compiler generated?
If small hot loops tend to be disproportionately hand written, and certain programs spend the majority of time in their hot loops, that could still be a decent percentage of time/instructions executed.