How can I watch youtube in Whonix Workstation?

YouTube works for me. There are no VirtualBox performance issues after enabling 3D Acceleration. This however is discouraged for security reasons in documentation. Was able to watch a video in HD as well as in 4K quality with Tor Browser inside VirtualBox.

Without enabling VirtualBox 3D Acceleration video viewing experience is degraded to to low graphic performance.

Added to documentation just now:

VirtualBox users note: Performance of videos might be slow. For a possible solution, see VirtualBox, Troubleshooting: Slow Video Playback.

Quote VirtualBox, Troubleshooting: Slow Video Playback.

Options:

What’s better or worse for security, enable VirtualBox hardware-accelerated graphics (3D Acceleration) or disabling CPU mitigations? Please consider resolving that question through Free Support Principle and let us know what you found out. Please use a dedicated forum thread for that.

Related:
VirtualBox 3D Acceleration