No, this is not expected. Can you share the project files?
There’s a benchmark in VTK tests which I’ve used to check for performance regressions. After you build the tests, run ./bin/vtkRenderingCoreCxxTests TestCompositePolyDataMapper -timeit and ./bin/vtkRenderingOpenGL2CxxTests TestCompositePolyDataMapper2 -timeit. Both should report nearly identical results.
Here are the results on Linux:
CPDM1:
CPDM2:
As you can see, CPDM1 is slightly faster than CPDM2, contrary to what you observed, so I think you may be using an older VTK.