head wind ( the longer you stay in it the more you suffer from it)
You can see your speed going up when you have strong headwind - the aircraft does it to reduce the amount of time you're exposed to that wind.
In opposite way, if you experience strong tailwind your mach number will be very low. As an example, yesterday flight was flown at Mach .64 @ FL350 with 50kt tailwind (CI=8 on an A319)
During descent, as Rick Studder said, it's better to fly near the best L/D ratio (green dot). The lower the cost index, the closer you'll be to green dot speed.
Getting to grips with fuel economy, published by Airbus, is a great literature for this subject. I think you may find it on smartcockpit.