I'm not sure what to conclude given the title of this post. Rewrites often improve things greatly. But a 2 hour time investment is not what anyone is talking about when they repeat mottos like "never rewrite."
It’s hard to assess to which extent the performance tweaking vs. rewriting tradeoff scales up. For a larger project, the rewrite could take a month; but maybe, diminishing returns helping, the performance tweaking would plateau short of a month.
So, vcat() instead of append!(). Darn. Faith in Julia restored. I’ll update the article.