EDIT1: There's a reason why there's a 1.2GHz gap between the base clock and the turbo clock. If the turbo clock was easily sustained then the gap wouldn't be so large (it's almost 50% of the base clock of the CPU). Also, 2.8GHz isn't the base clock for "power saving," because that's what C-States are for.
EDIT2:
GamersNexus has an article on this that shows the boost tables for CoffeeLake CPUs, including the 8400, and
here's a thread over on Tom's Hardware where TechyInAZ explains it. In so many words, the maximum boost clock is only achieved when you're using one core, and then it drops down from there based on the amount of cores that are being used, which is then based upon thermals.
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