I often see people in the gym slinging heavy weights around very quickly without regard to form or effectiveness. When concern for, “How much do you lift?” becomes an important factor (males of a certain age are often guilty of this), then often times form goes out the window in favor of the “how much” bragging rights.
You can lift more weight when you lift quickly because you are using momentum and gravity to sling the weight rather than controlling the weight with your muscles. It is also very likely that you are not getting a full range of motion when you lift this way. Unfortunately, many people lift incorrectly like this so they can lift a heavier weight. It actually defeats the true purpose of resistance training since lifting quickly will give you few results and increases the likelihood of injuries.
When lifting weights, check your ego at the door and lift with good form. This will give you more of the benefits you are searching for. Always lower the weight slowly and with a full range of motion. Do not drop the weight down (gravity working), swing your body, or bounce. A good way to know if your are lowering the weight slowly is to count to four during the lowering phase of the lift. You cannot lower a weight too slowly or control the weight too much, but you can get injured or get no results if you lower the weight too quickly. Always remember to let your muscles control the weight for safe, maximum results.
(Of course, power lifters and other specialists with very specific goals may lift more quickly when training, but should do so only with proper coaching and supervision.)