I'm very biased against “approximately X per Y” statements in general because just about any non-pathological curve has a linear tangent somewhere or the other, but if intervals wherein that proportional approximation can be said to hold are not stated it's pretty haphazard as a means of estimation.

Also, what kind of physical/logical process and attendant costs is it encoding? Does one per second of anything require one per second of something else? What if the processor were half- or double-the-bits?