Serious Lag due OpenGL Fragment Shader - ios

Question

Fragment shader causes serious lag when I run it on iPhone 4. I tried to comment part of calculations, however still there are some jitters even though I barely am doing any calculation in the Fragment Shader.
// Fragment Shader Code
uniform sampler2D texture;
varying lowp vec2 fragmentTexCoords;
uniform lowp float passAlpha;
uniform lowp vec2 inPosition;
uniform lowp float varUniform;
void main()
{
gl_FragColor = texture2D(texture, fragmentTexCoords);
lowp float disY = gl_FragCoord.y - inPosition.y;
lowp float disMax = 250.0;
lowp float coeff = 1.0 - varUniform;
gl_FragColor.rgb *= coeff;
}
//My render function is:
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP_TO_EDGE);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP_TO_EDGE);
glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
glFlush();
I am still not sure what could be the problem, I am sure iPhone can handle way more complex calculations ... Any ideas ?
Thanks in advance.

Answer 1

I would try this, it avoids several situations where you are doing calculations at enhanced precision because of the way your variables are declared... if this improves your performance, I can further explain why it works.
// Fragment Shader Code
uniform lowp sampler2D texture;
varying lowp vec2 fragmentTexCoords;
uniform lowp float passAlpha;
uniform lowp vec2 inPosition;
uniform lowp float varUniform;
void main ()
{
lowp vec4 color = texture2D (texture, fragmentTexCoords);
lowp float disY = gl_FragCoord.y - inPosition.y;
lowp float disMax = 250.0;
lowp float coeff = 1.0 - varUniform;
color.rgb *= coeff;
gl_FragColor = color;
}

Answer 2

Are you certain the stutter is caused by your fragment shader? Can you verify this by removing most operations from the fragment shader? I'm asking because you're really not doing anything too expensive in your shader code, and it would be odd for that to cause any performance problems. Are you sure you're not doing anything else, like uploading textures, in your update loop? What do the xcode profiling tools say about your performance?