A perusal of Google Maps indicates that typical rush hour traffic on the beltway is definitively worse than on I-95 between the beltways, in spite of the similar volumes. Particularly earlier in the week (on Thursdays and Fridays congestion on I-95 gets closer in severity to that on the beltway).
The obvious reason is that the beltway, especially west of the I-95 interchange, has generally inferior geometry - sharper and more numerous curves, shorter accel/decel lanes, short weaves that are not given C/D roads, tight loop ramps, narrower left shoulder, etc.