[alecmg]

maybe its a reference to Amdahl's law?

If you can parallelise 90% of your code, you get only 10x improvement even on infinite number of cores

[josefx]

You mix up time and code. You don't have to parallelize 100% of your code to scale with cores, you only have to parallelize the code that your software spends most of its time executing and that is often significantly less than 90%.