Canvas performance drops dramatically, when its size is (even by 1 pixel) bigger some limit

Please, look at this code.
It does almost nothing, just refreshing black screen 60 times per sec.
But it takes more than 50% of modern processor core (and with above average GPU)

Now, change canvas dimension from 1801x1801 to 1800x1800
Performance will improve significantly (2-3 times better).