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Kristian Van Der Vliet has implemented asynchronous input/output. This has been tested with QEmu, which shows increased performance both due to this and the also new implementation of memory-mapped files. A development build with async I/O is already available.
If we used a different kernel, we would have to re-implement everything we already have and then we would be at the mercy of a third party.
A software monoculture is bad. There is plenty of space for more than one kernel.