Function should_parallelize 

pub fn should_parallelize(num_elements: usize) -> bool

Returns true if the data size warrants parallel processing.

The whole question, not the size half of it. Without the parallel feature there is no pool to run on, so the answer is no whatever the size — and a caller that had to pair this with its own #[cfg] fallback was carrying the other half itself. One of them (process::apply_element_ops) then left its element count with no reader at all in a serial build.