Answering my original question about a small, lightweight Super Fast Charger for the S21: For home and hotel rooms I recommend the Rocoren charger (25w from one of 4 ports, 2.1" x 2.1" x 1.25", 3.55 oz). For laptops, and phones that take more than 25w, Rocoren makes a charger delivering 67w from one of 4 ports in the same size package, weighing 5.9 oz.
While in transit I will continue to carry the Samsung 15w travel charger (1 port, 1.39 oz). It's almost as fast as the 25w chargers from 20% to 50%, and essentially the same from 80% to 100%. It's smaller, lighter, and is white so easier to find rummaging through a daypack.
While testing, I learned what @Dinop43 indicated - Super Fast Charge is not that exciting. It only has a significant advantage when the battery is very low. Unless the Galaxy S21 is under 20% charge, the 25w Super Fast Charge is only moderately faster than the 15w Fast Charge. From 50% up there is essentially no difference.
In a test, charging the S21 from 50% to 100% took 42:27 for the 15w and 41:36 for the 25w, as measured by the meter. This graph shows why. It's only roughly accurate because all three chargers were read by eye during one charge from 9% to 100%, and the watt meter jumped around.
The 25w chargers only put out high power below 20% battery charge, and then dropped to below 16w. The 15w charger maxed at 14.8w but dropped off later than the 25w chargers. So during the measured test the 15w charger reached 90% about 3 minutes faster than the 25w. But the 25w caught up over the last 4%. I have no idea why.
