It is common in other engineering disciplines to assign budgets to the components of the system. For example, the engines of a VTOL aircraft are designed to provide a certain amount of lift, so the weight must be within a limit. At a high level, each part of the aircraft is given a portion of the weight budget which it should meet.
The reason this is done top down, rather than waiting until it's too bloated to get off the deck and then weighing each part and filing a bit off of the heaviest bit, is partly due to the cost of changing fabricated components. But a large part of it is that if you create a system where everything is a bit over budget everywhere, you can't just fix it in one place.
The classic software example is the SGI Indy Irix 5.1, which partly is why graphics intensive users have Macs and Windows machines now rather than SGI boxes.
"What's most frightening about the 5.1 performance is that nobody knows exactly where it went. If you start asking around, you get plenty of finger-pointing and theories, but few facts. In the May report, I proposed a "5% theory", which states that each little thing we add (Motif, internationalization, drag-and-drop, DSOs, multiple fonts, and so on) costs roughly 5% of the machine. After 15 or 20 of these, most of the performance is gone."
Frequently in discussions of performance, 5% is is said to be insignificant, and the advice is to wait until there is a problem and then look for a single bottleneck. For a large system, waiting until you have a problem may just lose you your main business.
