The next thing you can do is improve the gears. Your normal car runs faster in 5th than 1st right? And so will your Scalextric car runs faster with different gears. Gears change one rpm at the engine to a different rpm at the wheels. Now you can't be in the cars changing gears like your normal one so the so have one set installed. But you can pull them out and install a different set. If you're old gears meant that one revolution of the motor turn the wheels twice and your new configuration means they turn four times then you've just doubled your car speed! Be careful though, your motor only has some much torque/ power so if you gear it to highly the motor will not be able to handle it and your car won't make it up any slopes or worst yet not move at all. It may take a little experimentation to get it right for your motor.
So now you've ramped up your rpm but some of your power is wasted on friction. The only thing you want the motor to be doing is to be turning the wheels against the road (the bit where you want friction) but it also turns the axles and gears against their bearings, and you don't want to waste power here. So you need to reduce the friction as much as possible. You can do this by installing better bearings, which works great but is a little hard to source. You can also add lubricant the existing bearings and gears to reduce the friction. Or better yet, do both!
