See C.J.'sHeroic Sacrifice. Not only does he take out a whole mess of zombies by blowing up the bus, but the flaming wreckage effectively barricades the marina, allowing the others to cast off unimpeded.
Why do they waste so much time rescuing Nicole? Because she's in a gun shop and they're about to leave the mall. They can afford to waste ammo rescuing her because they can stock up once she's safe.
Additionally they were running out of ammo anyway and would presumably have relied on Andy to help them refuel once they got him.
Likewise Nicole watched how the zombies got into the shop. Notice how she parks the truck right beside the flap - to prevent the zombies from getting at it while she's climbing.
Why is Ana left in the mall while the others are rescuing Nicole, despite showing she's good in a crisis and she knows how to use a gun? Because she's a nurse and they'd need to rely on her to help with any injuries. They can't risk her getting infected.
It also helped to have someone more trustworthy than Steve as a backup plan for getting back into the mall.
Why didn't Vivian instantly attack when she came into Ana and Luis' bedroom? Look at her eyes, they are still greenish blue and clear, it's only after Ana throws her into the hallway that her eyes change bloodshot and sickly, she might've been still (only just) human before she attacked.
The zombie infection in this movie works much faster than most Zombie Apocalypse stories where it takes a day or two. This serves two purposes:
Combined with the fast zombies, it helpd to explain how the apocalypse spreads so fast.
Intentionally or not, it addresses a common bit of Fridge Logic in zombie movies; if the zombies eat their victims, how is there any body left to come back? In this case, it's because the transformation happens so quickly, the zombies don't have time for more than a few bites.
The fast global spread can also be addressed with the fact that those who are killed in the process of infection turn extremely quickly, with reanimation occurring within seconds of the death of the victim, but those that aren’t grievously or fatally wounded like Frank and Luda can survive for much longer periods of time, in Luda’s case days before they finally turn. As a result, while the infection can spread extremely fast from person to person and devastate crowded areas provided that their victims are mortally wounded or killed outright in the attack, it’s also entirely within the realm of possibility that anyone who only gained superficial injuries from an infected person will likely shrug it off as a run-in with a crazy person and go on with their day, potentially travelling great distances, like halfway across the country, or even taking a plane to another one, without incident before the disease finally kills them.
On another note, as seen from the news report special feature, it’s implied that the outbreak had been going on for a fair while beforehand. Notice how David Barrows has begun his report commenting that the ongoing supposed Madison race riots were not actually racially motivated after all. Unless entire crowds of people have died and turned all at once, then that means that the infection could have been lingering around for significantly longer than was previously thought, spreading under the radar and only reaching the news channel after large mobs of people have been infected and begun wreaking havoc on a scale that can be mistaken for rioting. Who knows how far it’s spread and how many people were infected before the outbreak was finally noticed.
The speed of the zombies is because they need to be, to offset their limitation, that their original counterparts don't have. In this continuity, zombification only seems to occur when bitten, in the original, anyone who died became a zombie, the bites simply being another, completely lethal, vector.
When Kenneth meets Ana, he has a shotgun on her and tells her to "Speak." Has he killed anyone "normal" who was simply too shell-shocked from the Zombie Apocalypse to speak?
