iOS OpenGL too slow

Question

I'm new to Xcode programming and I'm trying to create an iPhone game using OpenGL with support for retina display at 60 FPS, but it runs way too slow. I based it on the GLSprite example at developer.apple. I've already optimized it the best I could, but it keeps running < 30 FPS on the Simulator (I haven't tested it on a real device yet - maybe it's faster?). The bottleneck appears to be drawing the polygons - I've used really small textures (256x256 PNG) and pixel formats (RGBA4444); I've disabled blending; I've moved all transformation code to the load phase hoping for better performance; everything to no success. I'm keeping a vertex array that stores everything for that step, then draws using GL_TRIANGLES with one function call - because I think it's faster than calling multiple glDrawArrays. It starts lagging when I reach about 120 vertexes (6 for each rectangular sprite), but in many places I've read the iPhone can handle even millions of vertexes. What's wrong with the code below? Is OpenGL the fastest way to render graphics on the iPhone? If not, what else should I use?

OpenGL loading code, called just once, at the beginning:

glViewport(0, 0, backingWidth, backingHeight);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrthof(-1.0f, 1.0f, -1.5f, 1.5f, -1.0f, 1.0f);

glBindFramebufferOES(GL_FRAMEBUFFER_OES, viewFramebuffer);
glMatrixMode(GL_MODELVIEW);
glClear(GL_COLOR_BUFFER_BIT);
glLoadIdentity();
glMatrixMode(GL_MODELVIEW);
glLoadIdentity();

glBindTexture(GL_TEXTURE_2D,texture[0]); //Binds a texture loaded previously with the code given below

glVertexPointer(3, GL_FLOAT, 0, vertexes); //The array holding the vertexes
glEnableClientState(GL_VERTEX_ARRAY);
glTexCoordPointer(2, GL_FLOAT, 0, uvCoord); //The array holding the uv coordinates
glEnableClientState(GL_TEXTURE_COORD_ARRAY);

The texture loading method:

- (void)loadSprite:(NSString*)filename intoPos:(int)pos { //Loads a texture within the bundle, at the given position in an array storing all textures (but I actually just use one at a time)
CGImageRef spriteImage;
CGContextRef spriteContext;
GLubyte *spriteData;
size_t  width, height;

// Sets up matrices and transforms for OpenGL ES
glViewport(0, 0, backingWidth, backingHeight);
glMatrixMode(GL_PROJECTION);
glLoadIdentity();
glOrthof(-1.0f, 1.0f, -1.5f, 1.5f, -1.0f, 1.0f);
glMatrixMode(GL_MODELVIEW);

// Clears the view with black
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);

// Sets up pointers and enables states needed for using vertex arrays and textures
glVertexPointer(2, GL_FLOAT, 0, spriteVertices);
glEnableClientState(GL_VERTEX_ARRAY);
glTexCoordPointer(2, GL_FLOAT, 0, spriteTexcoords);
glEnableClientState(GL_TEXTURE_COORD_ARRAY);

// Creates a Core Graphics image from an image file
spriteImage = [UIImage imageNamed:filename].CGImage;
// Get the width and height of the image
width = CGImageGetWidth(spriteImage);
height = CGImageGetHeight(spriteImage);
textureWidth[pos]=width;
textureHeight[pos]=height;
NSLog(@"Width %lu; Height %lu",width,height);
// Texture dimensions must be a power of 2. If you write an application that allows users to supply an image,
// you'll want to add code that checks the dimensions and takes appropriate action if they are not a power of 2.

if(spriteImage) {
    // Allocated memory needed for the bitmap context
    spriteData = (GLubyte *) calloc(width * height * 4, sizeof(GLubyte));
    // Uses the bitmap creation function provided by the Core Graphics framework. 
    spriteContext = CGBitmapContextCreate(spriteData, width, height, 8, width * 4, CGImageGetColorSpace(spriteImage), kCGImageAlphaPremultipliedLast);
    // After you create the context, you can draw the sprite image to the context.
    CGContextDrawImage(spriteContext, CGRectMake(0.0, 0.0, (CGFloat)width, (CGFloat)height), spriteImage);
    // You don't need the context at this point, so you need to release it to avoid memory leaks.
    CGContextRelease(spriteContext);

    // Use OpenGL ES to generate a name for the texture.
    glGenTextures(1, &texture[pos]);
    // Bind the texture name.
    glBindTexture(GL_TEXTURE_2D, texture[pos]);
    curTexture=pos;

    if (1) { //This should convert pixel format
        NSLog(@"convert to 4444");
        void*                   tempData;
        unsigned int*           inPixel32;
        unsigned short*         outPixel16;

        tempData = malloc(height * width * 2);

        inPixel32 = (unsigned int*)spriteData;
        outPixel16 = (unsigned short*)tempData;
        NSUInteger i;
        for(i = 0; i < width * height; ++i, ++inPixel32)
            *outPixel16++ = ((((*inPixel32 >> 0) & 0xFF) >> 4) << 12) | ((((*inPixel32 >> 8) & 0xFF) >> 4) << 8) | ((((*inPixel32 >> 16) & 0xFF) >> 4) << 4) | ((((*inPixel32 >> 24) & 0xFF) >> 4) << 0);

        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_SHORT_4_4_4_4, tempData);
        free(tempData);
    } else {

        glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, spriteData);           
    }

    // Set the texture parameters to use a minifying filter and a linear filer (weighted average)
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MIN_FILTER, GL_NEAREST);
    glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_MAG_FILTER, GL_NEAREST);

    // Specify a 2D texture image, providing the a pointer to the image data in memory
    //glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, width, height, 0, GL_RGBA, GL_UNSIGNED_BYTE, spriteData);
    // Release the image data
    free(spriteData);

    // Enable use of the texture
    glEnable(GL_TEXTURE_2D);
    // Set a blending function to use
    glBlendFunc(GL_ONE, GL_ONE_MINUS_SRC_ALPHA);
    // Enable blending
    glEnable(GL_BLEND);
}

The actual drawing code that is called every game loop:

glDrawArrays(GL_TRIANGLES, 0, vertexIndex); //vertexIndex is the maximum number of vertexes at this loop

glBindRenderbufferOES(GL_RENDERBUFFER_OES, viewRenderbuffer);
[context presentRenderbuffer:GL_RENDERBUFFER_OES];

iOS OpenGL too slow

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