Cache the results of isCacheable calls to improve performance

[igorwulff]

This function call is relatively expensive when $elements has the full layout in it. Especially the PageCache module is calling this function a lot from the following places:

• Magento\PageCache\Model\Layout\LayoutPlugin
• Magento\PageCache\Observer\ProcessLayoutRenderElement

On my local machine I've seen the impact to be anywhere between 20-40 milliseconds, when the page has been loaded a few times to warm up cache (config, block, layout, etc...), but FPC is circumvented.

We could also opt to move this to the PageCache module, but I think it is ok to optimize this within the Layout class within the framework.

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Related Pull Requests

Fixed Issues (if relevant)

1. Fixes magento/magento2#<issue_number>

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1. ...
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• All automated tests passed successfully (all builds are green)

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[igorwulff]

@magento run all tests

[lbajsarowicz]

✅ The change looks good to me.

The method you're improving, originally did not look like a good example of code, however.

I was considering whether ?bool wouldn't be better than bool|null but there's no functional difference. I'm personally more towards ?bool