Re: Alternative title
True in general. However, it's not true that because you can't achieve optimal performance for a certain set of criteria that any improvement at all is impossible. There are a lot of ways in which a general purpose OS can reduce network processing overhead - virtual addresses and caches notwithstanding.
These include better use of scatter/gather features of the NIC or DMA controllers, careful control of allocation and copying and not shunting data to and from user space when the ultimate source or destination is another driver (eg streaming from/to a network to/from a file). None of these would render any other part of the operating system less usable. Nor do they require any form of "real time" response.