In general, classic race times may have decreased somewhat over the past 19 years, while the same cannot be said of the freestyle race. From 2008 on, the classic and freestyle events had separate start waves and a separate trails to OO. So while the distances for the two races were equal to each other up to 2008, in 2008 the classic trail was about 2 km longer, and from 2009 on, about 4 km longer than the freestyle trail.
Finish times, however, in this graph are standardized to a 55k classical race and a 51k freestyle race for all years, so the convergence of the finish times for the two events probably indicates that after 2008 classic skiers were able to increase their overall pace, especially during the first half of the Birkie, because of less interference with skate skiers. The downward slope of the trend line suggests that the classic finish times have decreased at a rate of about 2 3/4 minutes/year.
There were several notably slow races, particularly 2001 when 8 inches of new snow fell from midnight until midmorning on race day. In 2002, the classic race was plagued by warm conditions and a badly deteriorated track. In 2014, 18 inches of snow fell in the two days before the race, and the temperature was well below zero at the start.
Note: Finish times are standardized to a 55k classical race or a 50k freestyle race.