The Design Checker does not run by default. However, once you start it, it continues to run in the background until you close it. While it is running, the Design Checker will refresh its list of potential problems every 30 seconds.
While the Design Checker runs, Publisher's performance will be affected by the publication's file size. Larger files require more program resources to keep the Design Checker running in the background. If you want to improve performance, you can turn the Design Checker off by clicking Close Design Checker in the Design Checker task pane. You can start the Design Checker again if you want.