I do not know which Slash you have the XL5 brushed or the VXL brushless. It sounds like your setting it in one of the training modes those are there for beginners. If you want to reduce your speed put it back to 100% or in other words out of training mode. Then heres a couple of things you could do to reduce your speed.
1 Use a lower tooth pinion (better acceleration lower top speed) (loosen the slipper up a little to slow down the intial acceleration if its to much)
2 Reduce the voltage of the battery if your using a lipo or the 8.4v nimh then try to use a 7.2v nimh.
The stock TQ transmitter I believe only has steering trim adjustment. So like others have said if you really want to fine tune your driving than you should get a radio with EPA (Dual Rate , Expo) adjustments for steering and throttle.