You can't compare the flight times straight up like that and assume the ARJ is slower. The DC-9 time might be taken from the summer sked and the ARJ from the winter. Segment times vary due to jet stream/airport congestion, etc....look at the segment times for LAX-NRT in summer/winter and notice the variation of enroute time on the same type aircraft!

For those of you confused by Mach numbers, here is a simple rule of thumb.....take the mach number, say .70, and that works out to 7 miles/minute......Mach .80 would be 8 miles per minute....That is of course NO WIND. With no wind, an aircraft traveling at Mach .70 is going to cover 500 nautical miles in approx. 71 minutes (500 / 7 ). At Mach .75 (7 1/2 mpm) it would be 67 minutes. For most of the segments we fly in the smaller aircraft it's not too huge of a difference...
Now...throw in the wind. In summer the jet stream isn't too strong....Westbound we'll say 50 Knots. At a normal cruise of 420 knots (.70 mach) your 420 knots of ground speed would be reduced to 370 knots per hour. With a 120 knot jet in the face (winter) your ground speed is reduced to 300 knots. Going 500 miles, your previous trip goes from 81 minutes in the summer (50 knot headwind) to 100 minutes in the winter (120 knot headwind).
Now....throw in seasonality of traffic. Some airports are very congested at 6 in the morning during the summer....but not the winter. The ground time for your trip also varies by season. Dispatch will plan on say 12 minutes gate to takeoff in the summer, and 18 minutes in the winter...each airport varies, based on history.
Sorry to stray from the topic (MEM), but thought I'd pitch in on the speed/aircraft/segment discussion.