Pace of change is how quickly something changes. It also can be the pace of the change.
For example, in the video I noticed that Christianity was being rejected and that is was spreading slowly. Just as emperor Constantine was going to die, he was baptized. This caused the anti-christianity laws to be revoked. The pace of change of Christianity sped up because people saw that Constantine became Christian people began to convert too.
Another example that I saw was that the secret of silk was not shared so the romans had to go to China to get it. The pace of change was really slow because the Chinese didn't want to share the source of silk. It took many years for the secret to be revealed to the rest of the world.