How to improve OpenGL texture render performance - opengl

Question

I am using OpenGL to render text in my app, in my implementation, a glyph is a small texture, the render works correct, but the performance is too low. The following is my code fragment, when layout contains 8000 glyphs, it took about 200 ms to finish rendering (the render function), I think the "for loop" is the key to slow down the render process, perhaps render every single glyph as a texture is a “fool way” :(, Is there a way to improve the text render performance?
#define FONT_BITMAP_WIDTH 128
#define FONT_BITMAP_HEIGHT 128
void setupOpenGL() {
...
...
// create pixel buffer object to render texture
glGenBuffers(1, &_pixelBuffer);
glBindBuffer(GL_PIXEL_UNPACK_BUFFER, _pixelBuffer);
glBufferData(GL_PIXEL_UNPACK_BUFFER,
FONT_BITMAP_WIDTH * FONT_BITMAP_HEIGHT,
NULL,
GL_STREAM_DRAW);
...
...
// create texture
glTexImage2D(GL_TEXTURE_2D,
0,
GL_RED,
FONT_BITMAP_WIDTH,
FONT_BITMAP_HEIGHT,
0,
GL_RED,
GL_UNSIGNED_BYTE,
NULL);
...
...
}
void render(Glyph** glyphs, int count) {
// render every single glyph
for (int i = 0; i < count; i ++) {
// caculate glyph vertex position
float x0 = glyphs[i]->rect.origin.x;
float y0 = glyphs[i]->rect.origin.y;
float x1 = x0 + FONT_BITMAP_WIDTH;
float y1 = y0 + FONT_BITMAP_HEIGHT;
float positions[] = {
x0, y1, 0.0, 1.0,
x1, y1, 0.0, 1.0,
x0, y0, 0.0, 1.0,
x1, y0, 0.0, 1.0,
};
float textureCoords[] = {
0.0f, 1.0f,
1.0f, 1.0f,
0.0f, 0.0f,
1.0f, 0.0f,
};
// submit vertex and texture coordinate data
glBufferSubData(GL_ARRAY_BUFFER,
0,
sizeof(positions),
positions);
glBufferSubData(GL_ARRAY_BUFFER,
sizeof(positions),
sizeof(textureCoords),
textureCoords);
// submit texture image data
glBufferSubData(GL_PIXEL_UNPACK_BUFFER,
0,
FONT_BITMAP_WIDTH * FONT_BITMAP_HEIGHT,
glyphs[i]->bitmap_ref);
glTexSubImage2D(GL_TEXTURE_2D,
0,
0,
0,
FONT_BITMAP_WIDTH,
FONT_BITMAP_HEIGHT,
GL_RED,
GL_UNSIGNED_BYTE,
0);
// render glyph
glDrawArrays(GL_TRIANGLE_STRIP, 0, 4);
}
// present render result
openglFlushBuffer();
}