You are currently re-drawing the whole thing on every frame.

Instead you can draw it to a PGraphics once and just reuse the image.

Adapted Code

1. PGraphics gradient;
2.  
3. void setup() {
4.   size(960, 540);
5.   gradient = createGraphics(width, height, JAVA2D);
6.   gradient.beginDraw();
7.   for (int i = 0; i <= height; i++) {
8.     float inter = map(i, 0, height, 0, 1);
9.     color c = lerpColor(color(196, 254, 255), color(0, 58, 254), inter);
10.     gradient.stroke(c);
11.     gradient.line(0, i, width, i);
12.   }
13.   gradient.endDraw();
14. }
15.  
16. void draw() {
17.   image(gradient, 0, 0);
18.   frame.setTitle(int(frameRate)+"");
19. }

thx :)