I have a 2021 L405 AB. I rarely have the windows down, but noticed today that the front windows on both sides are both really slow to go down and back up. Is this because the windows are double glazed and so are really heavy or are the motored at fault in both sides?

They aren't really double glazed, the side, and rear glass is laminate like the windscreen, it's not really much heavier....

Slow windows could be a lot of things but usually something along the lines of use it or lose it, they just need using more often to free them up a bit....  Pete
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2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-

If it helps mines the same, it is covered under warranty as a known issue when it finally gives up. 2021 SVautobiography Dynamic Black ( Black wheels need to go )
1997 C140 Mercedes Benz CL500