Can you share the updated code and material.
So the cost of post process blur is now: 1000ms/54fps - 1000ms/45fps = 3.7037037ms. This is too cheap to cover extra pass overhead so don’t try to use render target with extra render pass. You could try to lower sample count even further by skipping samples using spatial and temporal dithering.
One small optimization is to pull out constant math from Custom node to material level so then material shader compiler can precompute those values at CPU level. Example: If you calculate inverse of Amount at material and then feed that to Custom node you save Division(which is more expensive than Multiplication). Material compiler can’t do any math folding for custom nodes and because Amount is Parameter it isn’t constant for shader compiler.
Edit: Notice that c is constant so you can move it outside of loop and just multiply blur value outside of the loop.