CPU as OpenCL device running in a sperated process?

I run a program (process A) on my Intel Xeon CPU E5-1620 v2 with two threads. One thread (1) starts an OpenCL application, that uses the CPU as device the other (2) does some calculations.

I noticed that the performance of thread 2, suffers from the OpenCL application execution of thread 1.

So I concluded, that the OpenCL application run by thread 1  starts a new process on the CPU (process B) and that process A and B get scheduled by the operating system. Because of this the performance of thread 2 suffers.

I could not find any documentation, that confirms my conclusion.

Is conclusion correct and more important, is there a documentation about it?