JS Optimization

After profiling, I see that roughly 25-30% of my program's time is spent inside this function.

Unfortunately, this isn't my algorithm, and I don't think I can improve it. The paper it came from is here: http://www.dgp.toronto.edu/people/stam/reality/Research/pdf/GDC03.pdf

My only option seems to be "performance tricks", but I don't see any openings. The only thing I can say for sure that I haven't tried is ``loop unrolling'', but I don't have much experience doing that myself, and I don't see how I could do it here while staying in bounds. If it helps, height, width, and iterations are all set by the user.

So: any ideas on how to improve the performance of this code?

function lin_solve(b, x, x0, a, c) {
        if (a === 0 && c === 1) {
            for (var j = 1; j <= height; j++) {
                var currentRow = j * rowSize;
                ++currentRow;
        var i = width - 1; do {
                    x[currentRow] = x0[currentRow];
                    ++currentRow;
                } while (i--);
            }
            set_bnd(b, x);
        } else {
            var invC = 1 / c;
            for (var k = 0; k < iterations; k++) {
                for (var j = 1; j <= height; j++) {
                    var lastRow = (j - 1) * rowSize;
                    var currentRow = j * rowSize;
                    var nextRow = (j + 1) * rowSize;
                    var lastX = x[currentRow];
                    ++currentRow;
                   
            var i = width; do {
                        lastX = x[currentRow] = (x0[currentRow] + a*(lastX+x[++currentRow]+x[++lastRow]+x[++nextRow])) * invC;
            } while (i-- > 1)
                }
                set_bnd(b, x);
            }
        }
    }

Canvas was a disaster. Not only does the typography render poorly, but the framerate was cut in half. Admittedly I used a pretty naive implementation, but after seeing what it looked like, it's not worth it to me to try and optimize for it -- not that I can think of many reasonable options for doing so, considering how simple my draw instruction is, and how much more I would need to, saw, implement redraw regions.

I tried using a <textarea> instead of a <div>, thinking that value, being of text only, might improve the performance. Alas, no improvement. Same amount of time in the renderer.

It's back to my only option of cleaning up the solver, then. I still have a lot of small places I can make optimizations. I'm also wondering if there's a place I could use Web Workers; I can't think of a place to fork, though.