result, effect, effectiveness
效 centers on the outcome of an action: the result, effect, or degree of effectiveness that follows from a cause.
效 combines 交 (exchange, mix) with 攵 (action, strike), suggesting the result or effect produced by an action. The exact historical development is uncertain, but the component relationship is broadly phono-semantic.
The left side 交 suggests interaction or exchange, and the right side 攵 is an action hand. Picture an action producing a visible result: the effect of your effort.
For コウ, imagine a cause producing a strong effect, like a loud 'ko' sound echoing as the result: ko -> コウ.