RC Model Reviews

To start, I know little to nothing about ground vehicles models anyway.

If you are happy you have eliminated the state of the battery the only 2 electric options left are the motor and ESC, have you tried swapping these out if you have access to spares.
Ahhh...... make that 3.........also check/re-do all the solder joints, I had a dry solder joint on my 450 heli and could not get enough rotor head speed.

this thread looks pretty old but is this a brushed RC? Us car guys are still moving into the brushless era and brushed motors can really loose power over time, the ESC's are ok besides the fact that almost every brushed ESC has no lipo protection, a lot of brushed users are starting to use voltage alarms so they can run a 2s lipo, I do not trust most NiMh batteries. Back to the topic another suggestion is to pull the motor and check the drivetrain, I think those three are going to cause problems more likely than soldering joints, electronics wise it's usually the motor or battery as with most RC's.

I had a similar problem with a EDF unit, it turned out to be the default end point adjustments on the throttle setup in the transmitter were not adjusted out to maximum giving lower than normal top end, I just maxed out the upper setting and now I have max speed.