[EnhancedButton] Speed up unmount

[fzaninotto]

• PR has tests / docs demo, and is linted.
• Commit and PR titles begin with [ComponentName], and are in imperative form: "[Component] Fix leaky abstraction".
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While profiling a moderate size page built with material-ui, I noticed that a significant amount of time is spent unmounting mui components. The culprit is EnhancedButton, which calls clearTimeout() in componentWillUnmount(). This represents 30% of the unmount time, about 25ms on a page with less than a dozen buttons:

It turns out that most of the time, the timeout that EnhancedButton wants to clean doesn't exist (it's the one created for the hover ripple effect). Adding an if before the call to clearTimeout reduces the time spent in EnhancedButton to something insignificant, and reduces the overall unmount time by 50%:

This may look like a small change, but as EnhancedButton powers many other components, it has important effects.

[oliviertassinari]

Nice finding 👍 .