So I encountered this issue, and found a solution and thought I'd share,
Problem: Very slow screen sharing when connecting to 10.8 or 10.9 machines without monitors. IE MacMini/Mac Pros.
Solution: Fake out the graphic card to think there is a display attached and issue resolved.
To accomplish this just grab three 100 Ohm resistors and a mini display port to vga adapter. Using the attached diagram connect the resistors to pins 1-6, 2-7, 3-8 and plug the adapter into the headless machine. external image link
Now when you remote in the lag is gone and all you have left to do is set the screen size to a workable resolution.
Hope this helps someone who was as annoyed as I was. (PS sounds like this is fixed it 10.9.2 though lol).
Gabe Shackney
Princeton Public Schools
