THE POSITIVE OF THE NEGATIVE
Throughout my life, I've been told to slow down and chew my food (I've always been a fast eater). The reasons make sense: less risk of choking and better digestion. As an adult I still am guilty of this (but thankfully am not corrected now, lol). This principle applies to training as well, but with a twist, slow down and feel your lift.
The negative (or eccentric) portion of a movement is when the muscle is lengthening. Opposite of this is the positive (or concentric) contraction, when the muscle shortens.
When you are training, studies have shown that the eccentric portion of the lift actually is more beneficial when it comes to muscle response. What that means is that if you don't maximize the negative portion of the lift, you're leaving progress on the table.
Additionally, your muscles are stronger in the eccentric portion of the lift, so if you use the same rep speed (or tempo) on this portion of the lift, it should seem easier (so another reason to slow it down).
Finally, if you aren't controlling your rep speed, typically form has a higher risk of breaking down, plus you get less mind/muscle connection. These two factors can at best, reduce the stimulus and pump you get but worst case is it increases injury risk.
The take home message is slow down your reps, I like 2-3 seconds concentric and 3-4 seconds for eccentric for muscle growth. Here is another example to slow things down to speed them up.