Edit: template previewing is now ⅓ faster for 6.6.

All results: https://docs.google.com/spreadsheets/d/1kQF-SKwanquvdBiKXiJYZKsviRoU256M_tf_-oDd95M

It's too early to tell how much we can do, I'll just list the PRs here as they get made.

So far we probably have a -40% for loading templates. I don't think we'll manage to improve other load metrics though.

This issue keeps track of all related PRs.

https://github.com/WordPress/gutenberg/pulls?q=is%3Apr+is%3Aclosed+label%3A%22%5BType%5D+Performance%22

Load (Editor/Preview/Pattern/Templates)

• Templates perf: resolve patterns server side #60349
• Block editor: optimise hasSelectedInnerBlock selector #60330
• Template part: avoid pattern fetch on mount #60297
• Performance: move style overrides to context #60382
• Pattern block: avoid fetching all reusable blocks on mount #60310
• Block preview: build in async rendering #60425
• Previews: avoid unneeded block selectors #60543
• Preview: skip rendering rich text #60544
• Drop zone: avoid media query on mount #60546
• Navigation block: avoid selector + style recalc on mount #60572
• Layout support: avoid two block editor store subs #60612
• Image cropper: remove clientWidth prop with useResizeObserver #60674
• Editor canvas: reduces resize listeners #60682
• useBlockSync: avoid replacing blocks twice on mount #60967
• Core data: getEditedEntityRecord: do not return empty object #60988
• useMatchMedia: cache queries #61000
• Block: remove outline related store selecting #61139
• Perf: batch block list settings in single action #61329