Okay, as I suspected, real FPS loss is a factor here. In for example, when running a game around 20-30 FPS, it's not unusual for there to be large, quick dips in the FPS. When running the game at 1-5 FPS, there can be skips as large as 30 pixels.
When setting to fixed rate 30, the frame rate isn't likely to drop like this, since you probably have more than enough power to maintain a consistent frame rate. But in the real world, dips like this can cause huge discrepancies.
So with a 'fixed' delta time, this isn't an issue and you can set a the frame rate and set minimal speeds and ignore sudden dips.
As for high speed objects, I doubt that'll be a problem, as long as the frame rate/delta time stays consistence.