利用for循环计算三维矩阵的平方和，其计算需要数小时如何解决？

我应该

f = X^2+Y^2+Z^2;
grid_x = ones(281,281,151);
grid_y  =   ones(281,281,151);
grid_z  =   ones(281,281,151);
for i = 1:151   
    X = grid_x(:,:,i);
    Y = grid_y(:,:,i);
    Z = grid_z(:,:,i);
    C(:,:,i) = vpa(subs(f),20);
    clear X;
    clear Y;
    clear Z;
end