Reduce the stopover time? You better should bring six days with you instead of six hours…
So you can only concentrate on the most importend "mainstream-sights":
Royal Palace - the historic Green Vault is a must (for the most)-but the tickets are for time slots and some days early sold out for the day-so use online-pre-booking to make sure to can see skd.museum/en/…index.html
If you are intrested in historic pompous armory, have a look at the brand new opend "Rüstkammer" inside the Royal Palace.
dont miss the "Fürstenzug" (all the historic saxon empereors) and directly on the backside of the wall it is on the "Stallhof" (historic horse riding place)
Zwinger-dont miss to go up to second level outside using the stairs in the side-pavilion and see the hidden baroque "Nymphenbad" an the Garden behind
go up to "Brühlsche Terrasse" to have a view over the river and see the steamboats-on backside of "Brühlsche Terrasse" look for the little bit hidden entry beside "Albertinum" at "Fortress Dresden" (historic catacombs and medieval city gate directly under the "brühlsche Terrasse") http://www.festung-dresden.de/de/startseite/
Cross the river and enjoy the best view on the historic skyline from one of the beergardens left or right down the bridge directly on the riverside, drinking a "Radeberger Pils" (oldest Pilsener in Germany). See the Golden Rider Statue and don't be afraid of the surrounding ugly estern germany architecture. Instead go one street ahead to the left (in looking direction of the statue) to the "Palaisplatz" and follow the "Königstrasse" into the only baroque quarter left after bombing of 1945. And if there is time enough, cross the "Albertplatz" at the End of Königstrasse" and go right around next ugly block to the "Alounstrasse directly into the "Neustadt" (New Town) with all its szenic shops and bars and young hip lifestyle.
If you arrive by train and its possible, better leave at "Dresden Neustadt" instead of "Hauptbahnhof" (Main Station) an go a few meters directly to "Albertplatz" to do the recommended way backwards. From Main Station better take a tram to historic old town to avoid waste of time walking through boring shopping streets to reach historic center. (take tram 8 in front of main exit -not city exit!- of train station in direction "Hellerau" and jump of at "Theaterplatz" after some minutes direct in the historic center)
Have a pleasant stay, despite the short time...