Say, While practicing, I am able to solve problems of rating 1200. It might take me 40 minutes or 2 hours or even 3 hours. But I am able to solve them by myself, without looking at any editorials.

Now my question is how do I improve the solve time? Should I solve harder rated problems(1300/1400) to improve the solve time of 1200 rated problems or should practice more of 1200?