It only has to compile on the fly one time. Also, you get a performance hit with any code the first time you run it because all the code no matter if it's pre compiled or not gets copied to a "shadow copy directory" (yes you can turn this off).

After pages and code files have been compiled the first time, the compiled resources are cached, so that subsequent requests to the same page are extremely efficient.  http://msdn.microsoft.com/en-us/library/ms366723.aspx