What is your point of reference when you say your car is slow? Are guys blowing past you on the straight or are the laptimes just off?
If guys are blowing past you and the drivetrain is pretty smooth, I'd have a look at your gearing. Every motor gears differently, so using someone elses or the manual's gearing, isn't necessarily right for your motor and track. Watch where the speed of the car tops out on the longest straight. If it tops out early and the car seems like it's lagging down the straight, increase one tooth at a time on your pinion until the speed tops out at the end of the straight. If it seems like you have more speed left as you let off at the end of the straight, go down a tooth until you top off at the end of the straight. Then, tune for the rest of the track. Also, check your timing on the motor. If you inadvertantly set the motor with negative timing, it will be slow and get excessively hot. If you're just sliding around and have to go slow to keep the car online, try laying the shocks down or use softer tires or inserts. Sorry if this is stuff you already know, but it could be one or more of those things that are causing the car to be slow.