Drawing/rendering is fast. Uploading bitmaps not so. IIRC there is not that much difference between blitting a GDI bitmap and opengl if you do it 1:1. Only if you start zooming, opengl becomes faster.
ugh.. it depends on the "GDI" implementation.
if GDI is pure CPU, OpenGL could still be a bit faster, since fillrate is typically higher on GPU. Plus CPU would be released for executing the code, rather than copying memory.
These days however, most of "GDI" are using hardware acceleration (through either OpenGL or DirectX).
Oh yes, if OpenGL drivers are not so good, it might be implemented through CPU as well
In this particular case (buffer: 0..919, 0..286) there won't be any difference.