It looks ACF blocks that are in patterns are not preloaded. Since patterns are loaded on every block editor edit screen, this is a big performance hit.

I can set an XDebug PHP breakpoint in my block template, save the block to a pattern, then load a new post with no blocks and see the PHP breakpoint processed. The block is still listed in the browser javascript console when I run acf.get('preloadedBlocks'), but I think the PHP breakpoint indicates the preloaded block is not used for loading the pattern. The pattern is still loaded with a new request to render the block.