[mh-cx]

I disagree. That's exactly the point: If you practice to achieve a constant typing rate (and start slow!) there will be no hard combinations anymore.

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.

[dasKrokodil]

But regardless of how much you practice, not all key combinations can physically be performed at the same speed, because some keys are easier to reach than others. So again, either you adjust all your typing to a constant rhythm and waste speed potential on the faster combinations, or you just accept variable speeds.

The comparison with musical instruments doesn't make that much sense because in music, the constant rhythm is required to stay in time. This requirement doesn't exist in typing.