As a user (IANAP), I think avoiding I/O contributes significantly.

The design paradigm of k is to memory map the data and then "forget about the filesystem"; transform data in memory, instead of reading and writing "files".

The slowest part of working in k, for me, is the process of reading files into memory. And I run kernel and filesystem entirely in RAM (mfs and tmpfs); no disk is involved.

If there are other software authors who also adopted this paradigm of avoiding I/O, I would like to know of them.