Time thread synchronization

Question

Hi all!
I'm developing an application that use a thread for load and draw some images. When the images are drawed, I set the thread state to sleep with the WaitOne method. Then, the application wake up the thread using the Set method. Now, during the execution, the application used about one second for wake up the thread. :S

By the theory, this should be a fast operation, but it isn't. Seem the thread run synchronously, and not asynchronously, as should be.

Why it takes a lot of time?
Have you any idea about this?

Thank you a lot!