Factors affecting the Efficiency of Shaders(GLSL)?

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C++

OpenGL

GLSL

My application simply loads an image as a Texture & rotate it in Z-axis.This is my code.
----Beginning of code----
Reference Source code.
http://www.opengl.org/sdk/docs/tutor...SL_Texture.zip
main.cpp
Code :

virtual void OnInit()
{
	glClearColor(0.0f,0.0f,0.0f,0.0f);
	glShadeModel(GL_SMOOTH);
	glEnable(GL_DEPTH_TEST);
	glEnable(GL_TEXTURE_2D);
 
	shader = SM.loadfromFile("vertexshader.txt","fragmentshader.txt"); // load (and compile, link) from file
	if (shader==0) 
	std::cout << "Error Loading, compiling or linking shader\n";
	else
	  {         
		 ProgramObject = shader->GetProgramObject();
		 locAngle = glGetUniformLocation(ProgramObject, "uAngle");         
	  }
 
	  // Create a Texture from file.
	  pTexture = cwc::TextureFactory::CreateTextureFromFile("Penguins.jpg");
 
	  if (!pTexture)
		 std::cout << "***WARNING: Failed loading texture!!\n";
}
 
virtual void OnRender(void)
{
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 
  if (pTexture) 
  {
	 pTexture->bind(0);
	glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MIN_FILTER,GL_NEAREST);	
        glTexParameteri(GL_TEXTURE_2D,GL_TEXTURE_MAG_FILTER,GL_NEAREST); // Disable Filtering!
  }
 
  if (shader) 
  {
	 shader->begin();
	 glUniform1f(locAngle, angle);         
  }
 
  if (shader) shader->setUniform1i("myTexture", 0);
 
 
	  glBegin(GL_QUADS);
		glTexCoord2f(0.0f, 0.0f); glVertex3f(-glXpos, -glYpos,  glZpos);//Left Bottom
		glTexCoord2f(1.0f, 0.0f); glVertex3f( glXpos, -glYpos,  glZpos);//Right Bottom
		glTexCoord2f(1.0f, 1.0f); glVertex3f( glXpos,  glYpos,  glZpos);//Right Top
		glTexCoord2f(0.0f, 1.0f); glVertex3f(-glXpos,  glYpos,  glZpos);//Left Top		
	  glEnd();
 
	  if (shader) shader->end();
	  glutSwapBuffers();	
}

Vertex Shader Source:
Code :

uniform float uAngle;
varying vec2 vTexCoord;
attribute vec4 position;
 
void main(void)
{
	mat3 rotation = mat3
	(
		vec3( cos(uAngle),  sin(uAngle),  0.0),
		vec3(-sin(uAngle),  cos(uAngle),  0.0),
		vec3(        0.0,           0.0,  1.0)
	);	
   vec3 projected_position =  rotation  * position.xyz;      
   vTexCoord = gl_MultiTexCoord0.xy;  
   gl_Position = gl_ModelViewProjectionMatrix * vec4(projected_position, 1.0); 
}

Fragment Shader Source:
Code :

uniform sampler2D myTexture;
varying vec2 vTexCoord;
void main (void)  
{  
    vec4 color = texture2D(myTexture, vTexCoord);      
    gl_FragColor = color;
}

----End----
Now my question is when i run this application in my PC it runs well(Spec:nvidia Geforce 210 GPU ,Processor:Intel 3 GHz)
But it consumes 50-55% of processor usage in my PC whereas it consumes only 2-8% of processor usage in some other PC's.
I've also tried to do the same Image Rotation using glRotate() & through custom Rotation matrix methods. it consumes only 0-2% of processor usage only.
How glRotate() consumes only 1 of processor usage?
What causes this increased Processor usage?
What are the factors which affect this GLSL performance?
Experts please help me on this. Thank you.