They knew perfectly well what caused drag.
Look at the world air speed record holders in the 1930s, the Schneider Air Races, and all the rest. The Germans, British, French, Italians, Americans all pioneered very sleek drag free designs that went very fast in the inter war years.
It was the 1940s, not the 1490s.
"Hell it took the RAF til 1945/46 to figure that giving the spitfire WHEEL DOORS would add 5mph."
-The Spit VII, VIII, XII and XIV all had wheel doors, the 109G-6, 109G-14, and 109G-10 did not. Fixed tail wheels were simply a design decision for combat a/c in WW2, some felt that the retractable tail wheel was just something else that could fail upon landing (which they sometimes did), and cause damage to the a/c. It wasnt considered that big a deal.
They did care about a loss of 5-10 mph, but as you say, bomb racks were required, as were DT racks, just as gondolas were, and they were used. They were combat a/c and drawbacks like that were just the cost of doing business. You cant fighter-bomb without racks to carry them.