The fast and slow is a relative term. It's not an absolute turm. If you move fast, your opponent will respond fast, and you have to respond fast too.I doubt anyone could train wrist locks full speed without severely damaging their partner. You can train against resistance but you'll quickly run out of students if they go flat chat.
If you try to train how to deal with your opponent's jab and cross combo, do you want him to throw a slow 1,2 combo, or do you want him to throw a fast 1,2 combo? If you ask him to throw a slow 1,2 combo, how long do you want to continue this kind of training? After you are good at dealing with combat speed combo, will you ever want to go back to the slow speed training?
If you want to train hip throw counter. When your opponent spin into you, you spin with him, interrupt one of his legs and take him down. Do you want him to move in slow speed so you can train your slow speed counter, or do you want him to move in combat speed so you can take advantage on his body momentum? If your opponent moves in slow speed, you won't be able to borrow his momentum, your counter may not work.
I don't like my opponet to throw a punch, freeze there, so I'll have all the time in the world to play my defense and counter. To me, to throw a punch and freeze in the thin air is bad habit. To deal with a frozon punch is also a bad habit. If I train this way, it will be neither good for my opponent nor good for myself. I want my training to have certain amount of "alert". If I don't concentrate, I'll get hit. I see no reason to reduce that level of "alert" even for the beginners level training stage. After a beginner has developed that "shirt on fire" feeling, he will be benefited by that for the rest of his life.