Are you trimming when you experience this slow down? The only slow down I've experienced is kind of an odd feeling, but ok. I'll be trimming and it slows down a tad them seems to burst with power back to speed. It's an odd feeling. All electric battery trimmers do this, to an extent. I sort of wish they just maintained a constant speed, regardless. What you are experiencing (if it is the same as what I get) is what I believe to be the built in protection circuitry that would shut down the motor if the string got tangled with something. Instead of jerking the trimmer out of your hand, the motor slows or stops.
Hope this helps