A positive side-effect of the recent collections update is that build times have improved by about 20 seconds. Prior to consolidating my posts into one _posts directory, regenerating the site took, on average, 60-67 seconds. Now, build times have decreased to 45-49 seconds. Apparently neither method of handling posts in multiple categories is ‘correct’ and both are acceptable, but if you have 1700+ posts across four categories like I do, a single _posts directory with subdirectories for each category is more efficient.