Too much Glossy Filtering
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As said in the [manual][1]:

> When using a value higher than 0.0, this will **blur** glossy reflections after blurry bounces, to reduce noise at the cost of accuracy. 1.0 is a good starting value to tweak.

You are using a value of 4.0, that means you are highly using the filtering for glossy bounces.

[![enter image description here][2]][2]

Use a lower values to lower the strenght of the blur applied.


  [1]: https://docs.blender.org/manual/en/dev/render/cycles/settings/scene/render/integrator.html
  [2]: https://i.sstatic.net/adhTc.gif