Images and HDRIs are blurrier in Cycles I don't know if it is an error or not, but images and particularly HDRIs (the blurriness is more pronounced in these) lose detail and become blurry in Cycles, when compared to the Material Preview, or in EEVEE (same results).

This is the image in Material Preview.


This is the image in Cycles.



It may not be very evident in the images, but when it comes to HDRIs, the whole sharpness is lost - even for a 4k texture. 
Why do you think this happens?
Increase viewport samples. and render it out. Maybe it's only because u r previewing the scene. The end result may be ok.