Treating the c5 to a good service can never be a bad thing. Making sure everything is properly adjusted & lubed should minimise the rolling resistance,which in turn leads to better range and performance, as does properly inflated tyres etc.
I didn't notice your particular c5 as being that much slower during our Cambridge run.
As an aside, don't forget not all c5's were equal even when new. Adam told me that the company experimented with the armature windings resulting in different power characteristics.
Some are slightly slower,but better at going up hills whereas others are slightly quicker, but then don't do as well on hills.
The only way to tell is to dismantle the motors and start comparing the armature windings.
I reckon a bit of service work will sort yours out nicely