It's hard to find a suitable speed for the zombies in my plays. If they're not slow enough, they're either not fast enough or too fast. I've made this suggestion in the official Discord, but I figured it'd be best to create a suggestion here.
I think it would be nice to have additional speeds for zombies for sandbox mode, such as:
Slow Shamblers: Much slower than the Shamblers, these zombies walk at a painstakingly slow pace in either idle wandering or pursuit of their targets. On their own, they're laughable and can easily be killed. In a pack, they're a fearsome threat, and even if you have all the time in the world, they'll catch up.
The best example of Slow Shamblers are the zombies depicted in George Romero films such as Dawn of the Dead and Day of the Dead. In these two scenes from Dawn and the opening sequence from Day, you can see just how slow the zombies truly are, practically shuffling their way to their next meal. Simplistic, dumb, and slow alone, but a true terror in numbers.
Joggers: A variant of the infected much slower than the sprinters, but much faster than the fast shamblers, joggers can still be outran but are more than capable of catching up to you almost right after. When idle or in small groups, they may not pursue quickly and be at the same speed as Slow or Default shamblers. If they are in bigger groups, they are more likely to start jogging after you. It can be easy to get cornered.
The best example of Joggers is the first and second seasons of The Walking Dead. As Rick enters Atlanta, the individual walkers that see him and follow after are extremely slow, like a Slow Shambler, and is easy to just avoid. However, when he runs into the horde a few blocks down the street, that horde breaks into a jogging pace, some faster than others, yet eager to catch him.
Here, Rick and Glenn are sprinting through a zombie infested street to get to the trucks a few blocks away, while a horde of walkers is jogging incredibly fast behind them, limping and struggling as they do so, but still able to pursue them. It is in this rare case that some infected are able to climb over the metal fence before inevitably knocking it down. It's as if the infected are far more confident than they are acting alone.
The current zombie speeds, while nice, still has some restrictions that I think more speeds like these could definitely fix. Slow zombies may be easy to deal with, but one mistake and it's over.