“Linux founder Linus Torvalds on Tuesday called for more regular performance tests on the Linux kernel, so that any reduction in efficiency can be highlighted sooner..
Currently, performance figures are only available for a few of the latest production kernels. Torvalds said it would be useful to continually test the performance of the development kernel, so that inefficient code can be spotted more easily.”
It seems that different versions of the kernal so a larger differential in performance tests. Check out the posting