It is possible that your anti-virus software scans every file in WoW which will make your game take a long time to launch.

This phenomenon occurs especially on BFA and Shadowlands and mostly with Avast.

Temporarily disable your anti-virus and launch your game, this may fix the situation with your game. If you want to continue to use your anti-virus during your game sessions, add wow.exe and if possible your game folder as an exception in your anti-virus.