"As for the descent angle. The VNAV path descent angle is similar to a glideslope, but it's by no means based on a shallow 3 degree descent angle. 3 degrees is pretty shallow for an idle descent."
Descent is influenced by Cost Index. The higher the Cost Index, the faster you will descend (Elevators used to maintain the path angle and throttles at idle... at least to the first restriction). If there are unforseen headwinds, the aircraft should come out of idle thrust to maintain the path.
Rgds.
Q.