I'm not sure what you mean by your laptop speed decreases but if you are referring to the cpu clock speed then that is normal. When being used very lightly turboboost will kick in providing a higher clock rate to the active core(s). As temperature/workload increases turbo boost naturally decreases or turns off to keep the cpu within its designed thermal envelope. Further heating up beyond the thermal envelope of the cpu will cause it to reduce clock speeds below the advertised clock speed and even turn off the pc/laptop if heat gets to be too much.