Specifics of actual human recovery ability aside, it's also not comparable because, while times do (generally, there are always exceptions) decrease from prelims to finals, I would attribute that more to the best athletes going out and then taking their foot off the gas towards the end once it's clear that they are moving on than them saying "I'll just go at 90% for this one and assume it'll get me through."
And since there is nothing at stake but advancement in that situation, not running the absolute best time you can in a heat only impacts you - unless you're dumb enough to slow up enough so as to not qualify, in which case you've just screwed yourself over while giving someone else a gift.