Size of render plays a part in the perceived sharpness.

If your HDR image is 4096 as above, FOV includes 614 pixels with 35mm focal length.

If your render size is 1228 pixels wide, it will have to stretch those pixels by 100%.

Unless you need the actual reflected image, you might just consider unchecking the "Visible in Camera" box on the Envsphere and compositing in background in postwork