fps drop on texturing

hi,

im having a problem with my program. so im trying to draw a viewport, save it as a texture, and draw it on another object. im planning to do a deffered shading and treat the texture as a buffer. i use the glCopyTexSubImage2D to perform this task. here is the sniplet of my code:

void drawOrbit() {
	
	glDisable(GL_TEXTURE_2D);
	glPushMatrix();
	
	glViewport(0, 0, (GLsizei)textureWidth, (GLsizei)textureHeight);
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	gluPerspective(45, (GLdouble)textureWidth/(GLdouble)textureHeight, 0.1, 5);
	glMatrixMode(GL_MODELVIEW);
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	
	glTranslatef(0.0, 0.0, -1.0);
	
	glBegin(GL_QUADS);
	{
		glColor3f(1.0, 0.0, 0.0);
		glVertex3f(0.9,0.9,0);
        glVertex3f(-0.9,0.9,0);
        glVertex3f(-0.9,-0.9,0);
        glVertex3f(0.9,-0.9,0);
	}
	glEnd();
	
	glPopMatrix();
	glEnable(GL_TEXTURE_2D);
	
}

void display() {
	
	glLoadIdentity();
	
	glClearColor(0.0, 0.0, 0.0, 1.0);
	
	drawOrbit();
	
	glCopyTexSubImage2D(GL_TEXTURE_2D, 0, 0, 0, 0, 0, textureWidth, textureWidth);
	

	//glDisable(GL_TEXTURE_2D);
	
	glMatrixMode(GL_PROJECTION);
	glLoadIdentity();
	glViewport(0, 0, (GLsizei)width, (GLsizei)height);
	gluPerspective(20, (GLdouble)width/(GLdouble)height, 0.1, 5);
	glMatrixMode(GL_MODELVIEW);
	glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
	
	glTranslatef(0.0, 0.0, -1.0);
	glEnable(GL_TEXTURE_2D);
	glBegin(GL_QUADS);
	{
		glColor3f(1.0, 0.0, 1.0);
		glTexCoord2f(1, 1);  glVertex3f(1,1,0);
		glTexCoord2f(0, 1);  glVertex3f(-1,1,0);
		glTexCoord2f(0, 0);  glVertex3f(-1,-1,0);
		glTexCoord2f(1, 0);  glVertex3f(1,-1,0);
	}
	glDisable(GL_TEXTURE_2D);
	glEnd();
	
	//glFlush();
	glutSwapBuffers();
	glutPostRedisplay();
	
	FPS();
	std::cout << gFramesPerSecond << std::endl;
	
}

its pretty simple: the main display method is display() and the viewport to be saved as a texture is drawOrbit. The performance shows 20 fps. When i disable the texturing (simply by commenting all glEnable(GL_TEXTURE_2D) commands, but still doing other task such as drawing the 2 viewports and copying it as a texture), the fps rise to 120. Is texturing that expensive? How can I trick it? in my program, i simply draw two object. is it normal to have such fps drop when performing texture?

thanks in advance