In layout, press shift-c; press p - if the camera panel is open, this will close it... press p again if so.
See Minimum samples? Leave it at 1.
Maximum samples... change it to ... oh, 128.
Change the threshold to 0.3 to 0.05; the former will be faster, the latter cleaner.
If none of this is available, tick on "Adaptive Sampling".
I looked at your image.
I gave you faster settings than quality.
Try these numbers:
Min Samples: 4
Max samples: 128
Reconstruction filter: Mitchell
Sampling pattern: Classic
Threshold: .01 to .001, with the latter being a lot longer to render.
See if you like oversample or not: .3 to 1
Try those numbers. Threshold and Max sample are your variables, in general, when rendering. The Min samples can be mucked around with but 1 makes for the quickest render. In this case, I said 4 because I figure you want all the stair steps gone and this will give the max sample a bit less to chew on, as it were.