Using Threading on Quad core speed up the code 65%?

Question

This sample code compares serial method with threaded method, on Quad core processor. The code just uses GetPixel() to read all pixels from 4 images. I found that the speed up is around 65%, why it does not equal 75% as I have 4 cores and all of them are fully utilized?

P.S:

Can you check the code as I do not do any I/O, and no other processes are working on the machines (normal windows processes)

Answer 1

It could be any number of things. A couple that come to mind

1. Overhead from administrating the different threads.
2. Other processes are using resources in the system at the same time.