A parking lot.
It’s super efficient as measured by cars / area of asphalt, but the throughput is terrible and good luck reacting to new information. You have to make that asphalt less efficient to make it useful.
Similarly, if your developers are at 100% capacity, they’re probably producing lots of code. But how much of that is shipping, and how able are you to react to new information?
Big thanks to Koushik Pal for the photo.