I'm still learning myself but there is an option in the render settings and then under environment that says Matte Fog, on or off. And I think it makes any objects in the background a little blurry like in your picture. But it could also have something to do with the noise degrain filtering or the post denoiser filter. I'm not sure what settings you are using, but I would recommend checking these first.
The Matte fog is a setting that is good during large outdoor scenes and you want some depth of field with far away objects fading away into the distance but I always keep it turned off for indoor scenes and smaller outdoor scenes. The noise degrain filtering and post denoiser can speed up rendering slightly I think at the cost of removing detail, so the picture will look a bit smoother and less grainy, but if you have high quality HD textures it will remove a lot of the HD details.
Other settings to check are the render convergence ratio, but also the max time and max samples along with that.
Max samples have a default at 5000, max time has a default of 7200 seconds (2 hours) and the rendering convergence ratio is default at 95%. What this means is that the render will stop after either of these conditions have been achieved, either 5000 samples have been done, 2 hours have elapsed or the convergence ratio reached 95%. So the key value is the convergence ratio, and you want that to be as high as possible for a high resolution image. 95% is good but you can bump it up to 98% for a better quality as well. The problem is the max time and max samples, because of either of those are reached before the convergence ratio reaches 95%, let's say it's only at 60%, then the picture will be grainy and not look good. So I usually set the max samples to 15000 instead of 5000, I set max time to 0 (infinite) and I keep the convergence ratio at 95%, this ensures that the render will be at least 95% complete before the render is finished. If you work on very large scenes you might need a higher sample number though.
I hope this helps!