Eating anything acidic ruins the absortion of amphetamines. Most drugs are absorbed more efficiently if you have a less acidic stomach. So avoid acidic foods like coffee, soda, popcorn, alcohol and meats if you really wanna get the most out of the drug. However coffee is also stimulating, and a lot of people use caffeine to increase the effectiveness of amps when they need to conserve. So to answer your question coffee does take away from the speed buzz and lowers the absorbtion of speed, but the stimulant effects of caffeine make it worth drinking anyway for some people. But if speed is easily accessable to you, then I would say to avoid coffee and just take more speed if you feel its not working enough. And if you really do need to conserve I would reccomend caffeine pills over coffee, as the coffee will cause a greater increase in the acidity of your stomach.
Taking an antacid, like Tums or even baking soda, will decrease the acidity (increase the pH) of your stomach, and thus increase the bioavailability/effectiveness of the drug. If you are trying to increase the effectiveness without needing to take tums or baking soda, then try eating more alkaline foods such as nuts, fruits and vegetables.