If you are driving at a speed of 60 mph (or kmph, it won’t matter) from one city to another and return at 40 mph, then what would be your average speed? A slowdown of 20% cannot be countered with a speed-up of 20%, it takes a speed-up of 33% to counter a slowdown of 20% (See note 2). In any aspect of life, once a slow-down has occurred, countering it is much harder without taking the risk of serious over-speeding and accidents.