did it ever run faster before? or has it always been slow?
some trucks come with different gear ratio's in the differential.
Even some with the small drive motor can make one run slower.
Battery capacity and charge level will affect how it runs too.

If it has always been this way that may be just the way it runs.

By the way
that serial number you gave is not for a E60XN-33
it is for a Z45N or E60Z probably
The E60XN serial number starts with a A268 sequence.