Yeah, don't think some people ITT know what slowrolling really means. People are really talking about 3 different things it seems: slowrolling, slowplaying, and using up the clock.
Slowrolling means you have the absolute nuts -- YOU CANNOT LOSE NO MATTER WHAT -- and are closing the action, i.e other player(s) all in, you're last to act, no other action can be taken after yours, and you take your time with calling. Or worse, you hollywood while stalling. Slowrolling should NEVER be done, unless maybe it's in jest between friends or something. It's really one of the worst breaches of etiquette in poker. Even when it's done to you, be the bigger person and don't do it in retaliation. There is really never a justification to slowroll anyone. Yes poker can be a cutthroat game, but there is still some lines that should never be crossed.
However some people think slowrolling is taking your time with a big hand that you'd almost never fold, but it's not the absolute nuts. This is NOT slowrolling in many peoples' view (and mine). This could be taking your time to ensure yourself that your hand is as strong as you think it is, or to take a moment to try and really narrow opponents range to be sure he can't nut you, or that you've actually read the board correctly (I've second-guessed myself here a few times myself), etc. Some think if you don't snap call a preflop shove with AA, it's slowrolling. It's not. You'll still lose ~20% of the time. And there can be some rare tournament situations where even folding AA preflop can be justified, so someone might take a moment to consider that before finally calling. Of course context is everything, so if someone is being an obvious jerk while doing this to tilt his opponent on purpose, then I think it probably crosses into the realm of slowrolling.
OTOH slowplaying is where you take a passive approach with a strong hand, like limping or calling, trying to keep players in the hand or to induce action from more aggressive players. This is a perfectly legit and accepted strategy, albeit often overdone especially by bad or beginner players.
Using up the clock to an excessive amount may be bad manners, but is not egregious like slowrolling. In small doses it might be used to create false timing tells
against certain opponents, like a form of hollywooding to act like you're unsure of your hand, again to induce certain action.