fast algorithm for drawing filled circles?

Just use brute force. This method iterates over a few too many pixels, but it only uses integer multiplications and additions. You completely avoid the complexity of Bresenham and the possible bottleneck of sqrt.

for(int y=-radius; y<=radius; y++)
    for(int x=-radius; x<=radius; x++)
        if(x*x+y*y <= radius*radius)
            setpixel(origin.x+x, origin.y+y);

Having read the Wikipedia page on Bresenham's (also 'Midpoint') circle algorithm, it would appear that the easiest thing to do would be to modify its actions, such that instead of

setPixel(x0 + x, y0 + y);
setPixel(x0 - x, y0 + y);

and similar, each time you instead do

lineFrom(x0 - x, y0 + y, x0 + x, y0 + y);

That is, for each pair of points (with the same y) that Bresenham would you have you plot, you instead connect with a line.