A secretary applies for a job; the interviewer asks her: "In your CV you claim that you can type 1000 characters per minute - for real?!" "Yes!", she replies, then adds in a low voice: "but such nonsense comes out..."
The bottleneck is often at the CPU. For a 10Ghz CPU, it can spend 1 cycle to process 1 byte. That’s the scale we’re at now.
https://www.tomshardware.com/features/ssd-benchmarks-hierarc...
A more complex case: a column store might write in batches. Later an insert in the middle might require the entire batch to be read from disk and then rewritten. This makes queries faster later on but at the cost of more disk io up front. In this case disk bandwidth is also saturated but write performance might be worse than an append-only log that does not optimize at all for reads/queries.