In fact it's exactly because of these "hard combinations" why your overall speed is slow, because you can't keep the high pace of the rest. So you have to master them first.
It's pretty much the same if you e.g. learn guitar: If you play a scale of notes you want to play it nice and smooth and not suddenly slow down at the hard part. If you can't play it smooth you're practicing to fast, so slow down until you mastered it.