Defer paper-item creation to first open for enormous performance improvement

[aletorrado]

Hi, I've noticed a big performance penalty just by having about 20 paper-swatch-picker on a single page.

After research, it seems related to paper-item initialization, so I just deferred the paper-item creation to the first open event.

Previously there was a very similar strategy in place, probably for performance reasons as well, where the styling was applied on the first open event, so the change that was made is pretty simple.

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