Is a faster implementation of an existing algorithm publishable?
If I understand the question correctly, the dilemma is whether to just distribute the code or whether to also make a scientific publication out of it.  The way that I typically think about this type of problem is to see whether it passes any of the following tests:


Does the improvement enable a significant scientific or technical work that was not previously possible?  For example, if faster simulation allows a control loop to be done in realtime that couldn't before, that advance may be scientifically valuable even if the methods are not interesting in and of themselves, but you have to demonstrate that value.
Does the improvement make a qualitative change in the operation of the algorithm which is interesting, e.g., changing a scaling property that was previously a limit?
Is the mechanism of the improvement interesting in and of itself, e.g., such that it teaching something about the nature of the algorithm or such that it might be applied to other algorithms or problems as well?


Any of these is a good reason to publish an improvement on an algorithm.