Tried that but its not that... (I dont think its a bug, its just the nature of how much and often the text it moving)
I got it to work really smooth like this:
1) just do it slowly across the screen where it is very smooth
(8 seconds was a good value @30fps)
2) in the editing software work out the math (ie to make it 2 sec instead of 8 ), keep all the frames from the synfig output, but just play it at a much higher frame rate (eg 120fps)
very smooth, size is not an issue because you can comress it by even 80% and the human eye wont notice the imperfections at 120fps
This technique can of course be used for anything not just text, for very smooth animation..