You may wish to try changing the option 3DRenderingEngine in the Option Inspector. I think you can also control this with style. eg
Style[SliceVectorPlot3D[{2 x, 2 y, 0}, {x^2 + y^2 - z == 0}, {x, -2, 2}, {y, -2, 2}, {z, -2, 2}],
RenderingOptions -> {"3DRenderingEngine" -> "OpenGL"}]
The choices vary by OS and their relative performance varies depending on your hardware.
