In answer to your question about time management: there will always be varying speeds in a classroom. Teachers need to go with the average speed: not too fast and not too slow.
I guess that you note time needed on your lesson plan; eg 10 minutes for a certain activity. I suggest that you note down the time that your students start that activity. 9 minutes later, you can take a quick look at the class. If it looks as if most of them are finishing up, you can end the activity at ten minutes. If nobody has finished, you can allow it to go over a couple of minutes, but not too long. You need to keep in mind the overall goals for the lesson and for the whole course.
Hope this helps!