How is the fragment shader optimized on the iphone?

Question

I want to port a example from Shadertoy to the iPhone with GLKit(Here).However, the effect is not very good when the program runs. The rendering effect is not smooth. Even the time interval was almost four seconds. I suspect it's because of the complexity of the operation in fragment shader. So I was wondering if there was a way to render without getting stuck, thank you.

fragment shader:

precision highp float;
varying highp vec2 TextureCoordsVarying;
uniform vec2 SceneSize;
uniform float elapsedTime;

#define TAU 6.2831852
#define MOD3 vec3(.1031,.11369,.13787)
#define BLACK_COL vec3(16,21,25)/255.

vec3 hash33(vec3 p3)
{
   p3 = fract(p3 * MOD3);
   p3 += dot(p3, p3.yxz+19.19);
   return -1.0 + 2.0 * fract(vec3((p3.x + p3.y)*p3.z, (p3.x+p3.z)*p3.y, (p3.y+p3.z)*p3.x));
}

float simplex_noise(vec3 p)
{
   const float K1 = 0.333333333;
   const float K2 = 0.166666667;

   vec3 i = floor(p + (p.x + p.y + p.z) * K1);
   vec3 d0 = p - (i - (i.x + i.y + i.z) * K2);

   vec3 e = step(vec3(0.0), d0 - d0.yzx);
   vec3 i1 = e * (1.0 - e.zxy);
   vec3 i2 = 1.0 - e.zxy * (1.0 - e);

   vec3 d1 = d0 - (i1 - 1.0 * K2);
   vec3 d2 = d0 - (i2 - 2.0 * K2);
   vec3 d3 = d0 - (1.0 - 3.0 * K2);

   vec4 h = max(0.6 - vec4(dot(d0, d0), dot(d1, d1), dot(d2, d2), dot(d3, d3)), 0.0);
   vec4 n = h * h * h * h * vec4(dot(d0, hash33(i)), dot(d1, hash33(i + i1)), dot(d2, hash33(i + i2)), dot(d3, hash33(i + 1.0)));

   return dot(vec4(31.316), n);
}


void main() {

   vec2 uv = 2.0 * TextureCoordsVarying.xy - 1.0;
   if (SceneSize.x > SceneSize.y) {
       uv.x *= SceneSize.x/SceneSize.y;
   }else{
       uv.y *= SceneSize.y/SceneSize.x;
   }

   float a = sin(atan(uv.y, uv.x));
   float am = abs(a-.5)/4.;
   float l = length(uv);

   float t = elapsedTime;

   float m1 = clamp(.1/smoothstep(.0, 1.75, l), 0., 1.);
   float m2 = clamp(.1/smoothstep(.42, 0., l), 0., 1.);
   float s1 = (simplex_noise(vec3(uv*2., 1. + t*.525))*(max(1.0 - l*1.75, 0.)) + .9);
   float s2 = (simplex_noise(vec3(uv*1., 15. + t*.525))*(max(.0 + l*1., .025)) + 1.25);
   float s3 = (simplex_noise(vec3(vec2(am, am*100. + t*3.)*.15, 30. + t*.525))*(max(.0 + l*1., .25)) + 1.5);
   s3 *= smoothstep(0.0, .3345, l);

   float sh = smoothstep(0.15, .35, l);

   float m = m1*m1*m2 * ((s1*s2*s3) * (1.-l)) * sh;
   //m = clamp(m, 0., 1.);

   vec3 col = mix(BLACK_COL, (0.5 + 0.5*cos(t+uv.xyx*3.+vec3(0,2,4))), m);

   gl_FragColor = vec4(col, 1.);

}