Why is a simple shader slower than the standard pipeline?

Question

I want to write a very simple shader which is equivalent to (or faster) than the standard pipeline. However, even the simplest shader possible:

Vertex Shader

void main(void)
{
  gl_TexCoord[0] = gl_MultiTexCoord0;
  gl_Position = ftransform();
}

Fragment Shader

uniform sampler2D Texture0;

void main(void)
{
  gl_FragColor = texture2D(Texture0, gl_TexCoord[0].xy);
}

Cuts my framerate half in my game, compared to the standard shader, and performs horrific if some transparent images are displayed. I don't understand this, because the standard shader (glUseProgram(0)) does lighting and alpha blending, while this shader only draws flat textures. What makes it so slow?

Answer 1

After long investigation, the slowdown of the simple shader was caused by the shader being too simple.

In my case, the slowdown was caused by the text rendering engine, which made heavy use of "glBitmap", which would be very slow with textures enabled (for whatever reason I cannot understand; these letters are tiny).

However, this did not affect the standard pipeline, as it would acknowledge the feature glDisable(GL_LIGHTING) and glDisable(GL_TEXTURE_2D ), which circumvents the slowdown, whereas the simple shader failed to do so and would thus even do more work as the standard pipeline. After introducing these two features to the custom shader, it is as fast as the standard pipeline, plus the ability to add random effects without any performance impact!