Speed Kills- the saturation and sharpness of your picture at least... Its a balancing of trade-offs in photography (and life-). Film Speed (ISO) is a value given to the sensitivity of the emulsions of traditional celluloid film to light. It is like the difference between how fast a lite skinned person will burn in the sun vs. a dark skinned person . The lower the film speed, the more light is necessary to saturate the film (dark skin), The image from a low speed film (50 or 100) has more time that it can be open to the light and saturate more deeply. Higher speed films don't need to be in the light very long before they burn. This is great if you are trying to capture an image in low light or a fast moving object. But just like the lite skinned person tends to freckle and burn, the fast ISO tends to be more grainy and less sharp. (Digital photography uses the same concept of ISO, it is just digitised). So if you are looking for deep saturation and sharpness use the lowest ISO you can. If you want to capture low lit scenes and fast moving things bump up your ISO accordingly.
Keep in mind that although film speed can kill saturation and sharpness, quality cameras and lenses can save the life of your image for sure. With modern digital equipment especially, you really get what you are paying for in terms of the quality of the processors and glass. (Ironically my old school Pentax K1000 with a prime 50mm (value $50) captures perfect color way more beautifully than any of my Canon digital equipment...(*I don't have the $20K set-up though).
So again the Philosophy of Photography teaches about balance and compromise. Seldom can we have it all but, by balancing the tools we are given, we can get what we are looking for. -Joah