Crashing is an effective schedule compression technique at times when a project is struggling behind schedule.
However, increasing the number of resources to twice the original number of the resources does not always cut the time by half.
This is due to:
A. Fast tracking the project
B. Risk mitigation due to crashing the project
C. The law of diminishing returns
D. Critical path analysis
Correct answer is C.
When one factor used to determine the effort required to produce a unit of work is increased while all other factors remain fixed, a point will eventually be reached at which additions of that one factor start to yield progressively decreased productivity.
This is commonly known as the law of diminishing returns. PMBOK, Page 197.