If more CPUs are applied to a parallel
program than can be kept busy given that program,
the excess CPUs are prevented from doing
useful work by contention.

Contention results in wanton consumption of CPU time
for BusyWaiting (perhaps in the form of SpinLocks),
and lots of
blocked processes and/or context switches for
BlocKing (perhaps in the form of
SemaphoresForMutualExclusion).
Either case results in poor SpeedUp.