The only time you will ever see consistent zombie speeds in a film or series is if it is running zombies. In those the actors are told to move as fast as possible.
When it is shamblers it is pretty much scene-to-scene.
In this case though, could just be that since they are just investigating a noise and not yet eying their feast, they aren't shamble-sprinting yet.
Zombie Novel Recommendations:
- Jesse Petersen's "Living with the Dead" series (comedic series)
- Mira Grant's "Newsflesh" trilogy (zombies are the cure for the common cold)
- Jonathan Maberry's Rot & Ruin (think the Walking Dead plus 15 years)
- Peter Clines's Ex-Heroes (What Marvel Zombies should have been)