I think the more compelling reason for "queries must be fast or slow, never 'both, depending'" is that it prevents engineers from accidentally using code originally built for reporting purposes (which might well have no latency requirements, or very relaxed ones) and putting it in a widget which gets slapped on people's home pages. You'd notice on your local, test, or staging instance "Hmm, my home page is taking 10 seconds to load -- that's bad" prior to pushing it live and, whoopsie, there's now 50 million people hitting a very scarily expensive query all at once.