In the cruise there will be a speed range within which you can safely operate and you will be flying at the upper end of that range.  

In turbulence the aircraft’s speed will fluctuate and to prevent the speed going above the maximum or below the minimum the aircraft speed should be reduced to the middle of the permitted range of speeds.  

Turbulence caused by convective currents reduces with altitude so climb, rather than descend, to get out of it.