Is there any speed difference if I use UMat and turn off GPU processing in opencv?

Question

Assume that I have a code which is written in C++ and uses Mat to hold images.

I want to change all Mats to UMat so the application runs on GPU as well on CPU (If I turn off GPU processing by calling ocl::setUseOpenCL(false);)

But I am wondering if I do this, is there any performance changes?

so in summery, if I have a code that uses Mat, if I change all Mats to UMats, is there any performance difference (When not running on GPU)?

Edit1

to clear the scope of question, Let me reword it:

why should not I use uMat instead of Mat assuming I am not using GPU?

Answer 1

When using UMat, opencv utilises Transparent API to use additional hardware attached. So, by turning off GPU processing you shouldn't expect a speedup.