[javiersr]

Something like:

gettimeofday(t0);
glTexSubImage2D( GL_TEXTURE_RECTANGLE_NV, 0, x, y, width, height, GL_RGBA, GL_FLOAT, (GLvoid*)data );
glFinish();
gettimeofday(t1);

and so on.

I'm writing and running more test right now to see if I can find something more concrete (may be something related to GL_NV_texture_rectangle, GL_NV_float_buffer, GLX_SGIX_pbuffer or any other of the extensions I'm using). I will post my result as soon as I get it.