Now, in 2.4, the Big Kernel Lock has gone away, everything is protected by its own little locks. This means the kernel is "reentrant". Now, the Preemptive Kernel Patch works by allowing a process to preempt the suspention of another process already in the kernel on the same [CPU]. This gives much better response, since if you have a high priority task that's blocked waiting on some data, and a low priority process running, as soon as the data is available to the high priority task it can resume running immediately, whereas before preemption it would have to wait until the low priority task left the kernel. 

  

 Surprisingly enough, in most situations this actually improves performance! The reason is that processes that have been blocked get an opportunity to run as soon as possible doing a bit of work and then usually blocking quickly afterwards allowing the machine to return to its previous job.