Yes, there is a performance hit but how large it is depends a lot on what you're doing.
"Switching" is one of the most costly operations and in Kernel mode you do not need to do it unless interacting with something in user space.. which you would only do because something in Userspace requested it somehow.
For other things, such as virtual memory, Microsoft found that the protections needed for virtual memory could be anywhere between 10 and 20%; but since there's no concept of virtual memory in kernel space: it's hard to say concretely that your program would be "20% faster". It would be too much of a different program.
