My 2012 Zero S was weak in the acceleration and could not reach the top speed on hightway.
A disassembly of the throttle showed that the sensor for the speed was loose and turned around when I opened for the throttle.
The sensor must not rotate!
The green arrow on the first picture shows the place to look
The sensor is hold in place by a plastic thing, and a black screw at the bottom of the throttle is holding it together.
If the screw is screwed full in to the hole the things inside is not sitting right.
The black screw is pressing on a plastic "thing" and that thing is holding the sensor.
On my first picture you can see the screw on my bike was not sitting right, and then the whole thing was getting loose and the bike lost it´s motor power.
The way you can check if your bike is ok is by turning the throttle handle, the bike should run just when you turning slightly on the handle.
If you almost has turned the handle in full position and it only takes the throttle in the end of the handle, then your bike has the problem.
Hope my little disassembly can help