Pace refers to the speed of the class. It is a subjective judgement, connected with how it feels for the learner to go through the sequence of activities in a class.
Some activities inevitably affect the pace of a class, for example, a whole group brainstorming activity will work best at a fast pace and an intensive reading at a slow one.
In the classroom
A pace that is either too slow or too fast can have negative impact on the learner. Simply asking learners if they feel activities went on for the right amount of time, and if they were too fast or slow will give a teacher good information about the pace of a class. Alternatively, another way to assess this is to ask another teacher to watch the class and give feedback.