Did airlines fly their aircraft slower in response to oil prices in the 1970s?
As a French who grew up in the Caribbean, I can tell that flights are getting longer. A flight between France and my island that took 7.5 hours 15 years ago is almost 9 hours long today.  



This post published by the MIT’s School of Engineering says (emphasis mine):


  Specified cruising speeds for commercial airliners today range between about 480 and 510 knots, compared to 525 knots for the Boeing 707, a mainstay of 1960s jet travel. Why? “The main issue is fuel economy,” says Aeronautics and Astronautics professor Mark Drela. “Going faster eats more fuel per passenger-mile. This is especially true with the newer ‘high-bypass’ jet engines with their large-diameter front fans.”  


Today's high bypass engines are more efficient at low speed:


  Observant fliers can easily spot these engines, with air intakes nearly 10 feet across, especially on newer long-range two-engine jetliners. Older engines had intakes that were less than half as wide and moved less air at higher speeds; high-bypass engines achieve the same thrust with more air at lower speed by routing most of the air (up to 93 percent in the newest designs) around the engine’s turbine instead of through it. “Their efficiency peaks are at lower speeds, which causes airplane builders to favor a somewhat slower aircraft”


This article also states that slower aircraft's structures are cheaper to build:


  “A slower airplane can also have less wing sweep, which makes it smaller, lighter and hence less expensive.” The 707’s wing sweep was 35 degrees, while the current 777’s is 31.6 degrees.