Both are significant. But the bit twiddling definitely shows up. There's some caching to avoid it in some situations, and breaking that causes measurable slowdo

Both are significant. But the bit twiddling definitely shows up. There's some caching to avoid it in some situations, and breaking that causes measurable slowdowns. In the end it's not too surprising - a good number of columns fit in a cache line, and adjacent lines can easily be prefetched. The cache miss matters a lot for the first column / row header, but not that much after.