Just YouTube - no other websites?
Desktop Hardware specs? (OS being Windows 1809.) What browser are you using?
Look in Reliability History/Manager and Event Viewer for error codes and warnings that occur just before or at the time of the freezes.,
Consider that having just done a clean install, Windows may be trying to do some updates to the current version.
Just leave the desktop be for a while to let the update process catch up and complete.
If the problem continues then use Task Manager and Resource Monitor to observe what the desktop is doing. Observe with and without YouTube.