Slowrolling is bad because on the river, when you're faced with all-in bet and your action is going to end the round, you either call or fold. If you're slow-rolling, aka hesitating or taking time to call with the best hand, then it's very out of manner. For example, let's say I have pocket Kings and you have rags and I flopped quads. On the river, you pushed all-in to bluff me, if I take time to call and then call, I'm pretty sure you'll be very angry.
I've slow-rolled few dickheads and retards in past just to piss them off intentionally. But other than that, you should never slow-roll people. I've slow-rolled idiots who bluff into an empty pot when I have the nuts, or slow-rolled dickheads who previously slow-rolled another person just to show how karma feels like.
NEVER EVER slow-roll unless if it's a person you have grudge against or previously pissed you off.
As for that hand Zeidman vs Harman, Zeidman got criticized because he was being a douche. When he was faced with an all-in bet, he had the absolute nuts aka straight flush. There's no reason for him to take his time to call. It should've been instant-call. If you're taking time to call on the river when you're last to act and you have the nuts, then you only make yourself look like a dick.