So is it only the dipper arm that is slow, walks straight, swings ok, when the arm slows down is it at the end of the stroke or soon as you put some resistance against the arm, did the shop put a pressure gauge on the pump and check pressure during this operation, what was the maximum relief pressure reading.
If you are operating the arm in and out does the engine start to lug on the arm out operation, also if you operate the arm in/ out while swinging does the arm speed increase.
These machines, as do most excavators merge the flow of the 2 pumps for quicker cycle times, if one pump is not not reacting, swashing up then you will get a reduced speed but if it is only the arm that is playing up then it will be a problem within that circuit.