Bottleneck

In the literal sense, this is the narrow part of a bottle. Not exactly a revelation there, but, uh...bear with us.

Figuratively, it refers to an obstruction that slows down a process. (Ever had an enema? Yeah.) For an engineer or designer, it is used to describe a phenomenon where the performance of an entire system is limited by a small number of resources. Something vital is missing and, until it is found and deployed, the system...just sits there like a Congressman.