JS Speed

A couple of things:

You don't need to create an array to send things out of an outlet.outlet(0, x, y, z) is just fine. this will get rid of the "fromsymbol" in the patcher as well.

The creation of the tempArrays is what is slowing down your code a lot (you are creating about 80 Array objects a second). Check the attached patcher for a better solution. At least the framerate is stable.

If performance is key I would rather create jitter objects from javascript code and connect them. They will always be more optimized than any js code.

this.autowatch = 1;
this.outlets = 1;
this.inlets = 1;

var step = 0.05,
    lastx = -2,
    lasty = -2,
    y = 0.5,
    borderx = -2,
    bordery = 0.1;

function bang() {
    lastx = -2;
  lasty = 0;

    this.outlet(0, "reset");
    this.outlet(0, "glcolor", 1, 1, 1, 1);
    for (var x = borderx; x  -2) {
            lineseg(x, y, 0, lastx, lasty, 0);
        }
        lastx = x,
        lasty = y;
    }
}

function lineseg(x1, y1, z1, x2, y2, z2) {
    this.outlet(0, "linesegment", x1, y1, z1, x2, y2, z2);
}