Why do subs sound bloated or slow?

The use of subs in 2 channel audio is controversial around A’gon. Detractors argue that subs usually make a system sound bloated or slow.

IME, the two biggest challenges for integrating a sub into a 2 channel system are optimizing frequency response and optimizing transient response. When frequency response isn’t flat, the bass sounds bloated. When transient response isn’t time aligned, the bass sounds slow.

Here is my pet theory about why systems that use subs often sound bloated or slow: Under many circumstances, optimizing frequency response and optimizing transient response is a zero sum game. In other words, getting one right usually means you get the other wrong.

Thoughts?

Bryon