Question Graphics.SetRenderTarget(RenderTexture) takes way too long on GPU

i am working on rendering a custom mask map just before the frame start rendering.
in my first attempt i chose:
RenderPipelineManager.beginCameraRendering += RenderMask;
within in the
private void RenderMask(ScriptableRenderContext context, Camera camera) {}
- i get a temporary render texture
- and set it as render target using Graphics.SetRenderTarget(RenderTexture);
- and render a mesh using Graphics.DrawMeshNow();
according to the unity profiler this takes 0.26ms on the GPU.

going with a CustomPass instead and using RTHandles, CoreUtils.SetRenderTarget() and HDUtils.DrawRendererList() does not give me any significant costs for setting the render target at all: 0.000ms.

i would not expect this discrepancy and wonder if i do something completely wrong...