Haven't checked with 2.19, but I too think it's probably SWScale, as I'm guessing this happens when ffms2 has to convert colourspace (which, with 10bit input, means always). I'll be mean and say I don't particularly care about it, though, since I've already switched to L-Smash Works...
I definitely think L-SMASH-Works is better, but the lack of publicity really is hindering its adoptions rate. I probably wouldn't have discovered its existence had it not been for mirkosp telling me about it.