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 mixer.

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