It might not be the OS, but just statistical inevitability. If you're talking about CPU utilization on Linux, for example, it's not all that unlikely that the number you're staring at isn't "time spent by CPU doing things" but "average CPU run queue length". "100%" then doesn't only mean the CPU gets no rest, but "there's always someone waiting for a CPU to become free". It likely pays off to understand where the load numbers in your tooling actually come from.

Even if that weren't the case, lead times for tasks will always increase with more utilization; see e.g. [1]: If you push a system from 80% to 95% utilization, you have to expect a ~4.75x increase in lead time for each task _on average_: (0.95/0.05) / (0.8/0.2)

Note that all except the term containing ρ in the formula are defined by your system/software/clientele, so you can drop them for a purely relative comparison.

[1]: https://en.wikipedia.org/wiki/Kingman%27s_formula

Edit: Or, to try to picture the issue more intuitively: If you're on a highway nearing 100% utilization, you're likely standing in a traffic jam. And if that's not (yet) strictly the case, the probabilty of a small hiccup creating one increases exponentially.