That's the right idea - losing speed increases your travel time more than gaining speed decreases it.
Here's a driving example:
Suppose you're traveling 60 mph on highway and hit 10 miles of 40 mph construction zone
You spend 15 minutes there instead of 10, losing 5 minutes.
Then suppose you try to get the time back by going the next 10 miles at 80 mph. Bad strategy, for two reasons.
First, that next 10 miles still takes 7.5 minutes, saving not 5 minutes as you hoped, but only 2.5 minutes.
Plus, you got a ticket.
In the plane example, it's also useful simply to calculate travel time intervals:
Suppose each leg of the trip is 600 miles, the air speed is 150 mph and the wind is 50 mph.
At 100, 150 and 200 mph ground speeds, the one-way travel times are 6, 4 and 3 hours, respectively.
Without wind, the total time is 4 + 4 = 8 hours. Avg speed = 1200/8 = 150 mph.
With wind, it's 6 + 3 = 9 hours. Avg speed = 1200/9 = 133 1/3 mph.
So the percentage change in speed or travel time is 12.5%