Using vtkRenderer and vtkOpenGLRenderer

Hi there, I upgrade VTK8.2 from VTK8.0 and my project successfully compiled and run. But I found something interesting, like when just replacing libraries, the CPU memory usage is increasing from 7-10% to 20-30%, and the frequency went very slow sometimes.
Then I tried to put these things to GPU to speed up, with changing the Renderer from vtkRenderer to vtkOpenGLRenderer, the same as the vtkRenderWindow.
Then the CPU usage decrease back to 7-10%. But the GPU usage didn’t change much, like about taking 20-40% around.
How could that be?