Usually noticed when a dozen or more complex drawings are opened in AutoCAD or AutoCAD Mechanical and they are closed one at a time with the red x at the top right or from the drawing tabs, it takes a while to close them all out.
This method to close drawings is fine for several drawings, but tedious for many open drawings.
Though not obvious, using the CLOSEALL command or perhaps the CLOSEALLOTHER command will close all open drawings much faster.
If some or all drawings need to be saved, also use the SAVEALL command before CLOSEALL.
Note: If this command is frequently needed, then use the CUI command (Customize User Interface) to locate and drag the Close All command to the Quick Access Toolbar at the top left of the drawing screen, like this:
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