I can tell you that the same zombies you mentioned turbo on 60fps aswell sometimes, from watching 120fps streamers I am convinced it happens just as much at 60fps.
Zombies turbo on all frame rates. I'm not to positive on this (correct me if i'm wrong), but assisted either has no or very limited amounts of turbo-ing, compared to standard which has a few zombies that turbo compared to hardcore. I don't run assisted, i've only ran Hardcore and Standard. But i'm almost certain that zombies turbo from RNG. It could also be environmental factors, like bumping into something, causing them to stumble etc.
Standard and hardcore both have fast zombies, the difference is they start to turbo after a certain da has been reached on standard. After that they will also double lunge on standard. I believe its da 6.
The Asian zombie after Elliot in rpd1 sometimes turbos on standard with 4 DA so I'm pretty sure DA doesn't affect turbo zombies.
