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);
            }
        }
    }

When you assign to innerHTML, the browser probably reflows the page, which is a comparatively slow operation.
Canvas, or WebGL if you want to get fancy, could be a lot faster.
WebGL in particular would let you upload a raw array and let the shader do the work.

Is this for some HTML5 demonstration, or something that should run on smartphones?
If none of the above and you just need something that runs in a browser, as much as it pains me to say it, you'll get better performance by making it a Java applet.
That's not to say I recommend it if you can get at all acceptable performance by doing it `browser native'.