One of the things I enjoy about Brazilian Jiu-Jitsu is being able to fight as hard as I can, with a competitive opponent and have no injuries or ill will after the roll. With most martial arts if people are going full speed the participants will be much more likely to get injured. That being said, slowing down is one great way to become better at BJJ.
It is common for new BJJ students to rely on speed and strength to make up for their poor technique. The student will have an idea of how the technique works and they will force it on their rolling partner. This is a recipe for learning Jiu-Jitsu the hard way. You will have many hard fought battles without learning much.
Rolling slower is less tiring and you get to roll longer. Many people get tired after just 5 to 15 minutes, this makes it hard to get a lot of “mat time”. Just by slowing down and controlling your breathing you will be able to roll much longer. The longer you roll the more your body can learn the subtle movements of BJJ.
It is common for new students to say “if I slow down I will get my butt kicked”. That is a good concern, you will likely be playing more defense that you typically do with people of similar skill. That’s great, defense is very important. The more advanced students are likely to notice you slowing down. Because you are not going so hard the more advanced students will probably slow down.
If you can do a technique slowly and without much power you can do it with speed and power. Being able to do submissions slow on a resistive opponent is a good sign that you are doing the move correctly. Your speed and power will be there for you should you really need it.
Another way people roll slowly is to suggest to the other person that you both roll at an easy pace. When you are rolling at an agreed slow speed don’t get competitive and speed up over the course of the round. Sometimes people will take turns doing submissions and escapes. For example the first student does one or two moves and the other student escapes and then does a couple of moves. This is also great if you are nursing an injury and not ready for a full speed opponent.
It may be hard at first to slow down but in the long run it will help you get better at Brazilian Jiu-Jitsu.