I think stiff does not quite nail it - you have to play somewhat more straight in these time signatures, especially if the tempos are fast. In slow to mid tempos, you can superimpose a triplet feeling, but in fast tempos this is getting dangerous. It's a different rhytmic conception coming from musical cultures not knowing the concept of superimposing two different pulsations, but use one of them at a time. Otherwise the subdivisions are obscured an you are risking to get lost in the rhythm.