I beta tested it and it was perfect. Now that it's released there's a severe lag in the brush strokes. Certainly they'll fix it, but don't upgrade for now.
You are not the only one to have this complaint Sam. Several people on the Procreate Forums had similar problems. I have not had any problems with lag but I did have some problems with a few brushes not staying the size that they were set to. That problem seems to have stopped. Several people said that many of the Procreate issues occurred only after Apple released IOS 11 to the general public. Have you updated to IOS 11.0.2 yet?