EDIT: But it could also be that your computer is somewhat faster than theirs? Do you happen to have some very fast CPU? Can you say which? When I run C-like C++ versions of your code I get the speeds you get with node.js. However, you made overall much better results than they were able, it's still great work!

    #include <stdio.h>
    int main(int argc, char* argv[]) {
        enum { height = 4096, width = 4096 };
        unsigned* a = new unsigned[ height*width ];
        unsigned* b = new unsigned[ height*width ];
        if ( argc < 2 ) { // call with no params
            // to measure overhead when just allocations
            // and no calculations are done
            printf( "%d %d\n", (int)a, (int)b );
            return 1;
        }
        if ( argv[1][0] == '1' ) // call with 1 the fastest
        for (unsigned y0 = 0; y0 < height; y0 += 64)
            for (unsigned x0 = 0; x0 < width; x0 += 64)
                for (unsigned y = y0; y < y0 + 64; y++)
                    for (unsigned x = x0; x < x0 + 64; x++)
                        b[x + y*width] = a[y + (width - 1 - x)*height];
        else
        for (unsigned y = 0; y < height; y++)
            for (unsigned x = 0; x < width; x++)
                b[x + y*width] = a[y + (width - 1 - x)*height];

        return 0;
    }