The SVP Shader for blocks has a tool tip saying that it runs faster if it's run only on the cpu, or is that separate from the rendering device?
If you have appropriate GPU (HW-acceleratio is enabled in SVP) then I recomend to select GPU or "Do not change" in option Rendering device. It will give you more quality of interpolated frames.
SVP can work without the use of GPU, but this reduces the quality of intermediate frames calculation and significantly increases the requirements for CPU performance; so, using a GPU is highly recommended. Its power is not especially important (for example, a GPU integrated into Intel Core 6th gen. processors, is sufficient for calculating intermediate frames up to 4K@60 fps.