[Gaz64]

I'd say up to about 3000 rpm, the first 10% of the burn time takes a fixed time period hence the need to advance the timing for increasing rpm.
After about 3000 rpm; the cam, cylinder head design, compression, intake, exhaust combo of the engine all start to work in harmony, the engine is breathing better, has higher volumetric efficiency, has higher cylinder pressure and turbulence, faster flame front across the chamber, so does not require any more advance.

Some engines like more than others and vice versa, some like fast curves, it depends on chamber design, CR, fuel, etc.