I'd say it's particularly rude not to respond promptly (or even notify them in advance), if the slower recruiter/company had initially correctly set the timelin

I'd say it's particularly rude not to respond promptly (or even notify them in advance), if the slower recruiter/company had initially correctly set the timeline expectations. It's one thing if they say "you're the first candidate we spoke to, so we'll need a couple weeks before we get back to you with a decision" and another if they say "we'll let you know in a few days" (or set no expectation at all).. and then say nothing until 2 weeks later. In the latter case, a reasonable candidate would assume a tacit rejection, as has been common in the industry, and could be forgiven for forgetting about that company entirely, even a week later.