Usually uC speeds are constrained by the Flash fetch and decode, and so that is what they 10MHz applies to.
 In some cases, you can clock certain peripherals faster than the core and there you can have some parts running faster MHz than the core, but that ability tends to not be on the smaller uC.

Hi all,

just looked through the ATTiny 861 datasheet with the "new" options to
use a High-Frequency PLL clock with the 8 MHz calibrated oscillator to
clock the processor and the periphials. So by using this combination,
the processor can run at 16 MHz with the internal RC oscillator.

However, the datasheet also says that the maximum "speed" at 3.3V is 10
MHz. It doesn't say what this "10 MHz" is referring to. Is this the
maximum speed of an externally attached clock signal (XTAL or external
clock), is it the maximum CPU clock, the maximum IO clock, ...? Or, in
other words, is it possible to use the High Frequency PLL clock for the
CPU at 16 MHz with a 3.3V power supply?

Holger