We talk about big-O complexity a lot when talking about things like this, but modern machines are scarily good at copying around enormous linear buffers of data. Shifting even hundreds of megabytes of text might not even be visible in your benchmark profiling, if done right.
When benchmarking, I discovered that the `to_pos`/`to_coord` functions, which translate between buffer byte positions and screen coordinates, were by far the heaviest operation. I could have solved that problem entirely simply by maintaining a list of line offsets and binary-searching through it.