blurry line while playing

[NeatNit]

If my understanding is correct, vsync does not ever slow your PC down. It simply means that your computer doesn't do worthless extra work when in fact you won't see any improvement from it. Vsync means that your GPU won't create more images than your monitor can show in one second. Technically it's slowing your GPU down, but that's because if it didn't then all the extra work would be 100% wasted.

If someone thinks this post is wrong, please correct me. And please someone explain to me why vsync isn't enabled by default, or even forced, on most games...