It takes a long time for Serato DJ Pro to show the virtual decks after I launch the application.

When multiple ASIO drivers are installed or multiple audio devices are connected to your PC/Mac, it might take a long time for Serato DJ Pro to recognize the DJM-S5.

Disconnect audio devices other than the DJM-S5 and uninstall unnecessary ASIO drivers and virtual audio devices. Visit the website of the relevant manufacturers for details of how to do this.