As neither motors nor ESC are getting hot, the first suspect has to be the battery, either not getting fully charged, or not holding a charge. I would check the voltage at the battery terminals. Fully charged, you should see over 13.5 volts. Swich on and spin the motors, the reading should not change by more than a very small percentage. Or apply a test load such as a car headlamp bulb.
For motors, a couple of 15 volt 545s are a fairly cheap drop-in replacement, and should have a more suitable power delivery. Looking at a web search for rs540sh, it seems a popular stock motor for cars - it is intended to rev freely, whereas you are needing torque.