Gym trainer said I do my reps too slowly

As long as you have good form slow is better. But its also alot more difficult.

Can you do slow pull ups?

Depends what the exercise is. If you're doing a circuit and it's 'burpees for 1 minute', only doing two is probably not ok 🤣

Its actually good to do it slowly and controlled and feel the stretch. Instead of rushing through the set.

yes there is such a thing, I was always too slow with it all, I don't think there's any better or worse, just follow your ways, listen to the trainer of course but most importantly listen to yourself

Most is timed