I am not a believer in teachers "slowing down" their English to aide their students to learn.
I see this to be a hindrance.
I believe in learning by way of immersion. By constant listening exercises, your brain becomes accustomed to English being spoken at the "regular" speed.
In my opinion, the best way is to articulate and ensure a teacher speaks clearly.
Instead of using a dictionary, I find it the most useful way to explain a word or phrase a student does not understand is to simply use other English vocabulary. Using a dictionary ot translator should be more of a last resort.
One of my English classes at Lycée Corbon in Paris 75015