Comment by MindSpunk

The triangle density of even a basic font is crazy high at typical display sizes. All modern GPU architectures are very bad at handling high density geometry. It's very inefficient to just blast triangles at the GPU for these cases compared to using an atlas or some other scheme.

Most GPUs dispatch pixel shaders in groups of 4. If all your triangles are only 1 pixel big then you end up with 3 of those shader threads not contributing to the output visually. It's called 'quad overdraw'. You also spend a lot of time processing vertices for no real reason too.