DennisOSRM/Project-OSRM — comment on commit e83891b4fc0a7fbf105c1ae1f5f6cafd32e74dd8
File:  line 0
By DennisOSRM:

The time needed for routing is usually neglectable. Most of the time ist spent in I/O and annotating the geometry and computing turn instructions.

One I/O is made for each location (start,dest,vias). This takes a few milliseconds. The more locations, the more this dominates. The hinting mechanism addresses this situation. Usually, new via points come one-by-one and also if you are dragging a route, it's only one location that changes.