There does seem to be an issue with the severe drops in pace players suffer after only a few overs. Players tend to lose their zip too quickly, and it just doesn't seem to come back during an innings.
I've seen Harris down to below 120 as his very peak after only 6-7 overs, and it doesn't come back even going into a new day. Compare this to Harris even in the last match he played (carrying a knee injury) he was hitting perfect 140 kmph yorkers in his 25th over with less than 5 overs to play on day 5.
Losing a bit of pace is one thing, but maybe 10-15 kmph over 10-15+ overs, not 20-30 over 5-8. I guess the risk of injuring your key bowlers would spice up the battle between maintaining pace and managing fatigue, but at the moment commanding Johnson and Harris as a pace attack may as well be a pair of mediums.