Depending how how spread the load is amongst the threads (that is, whether the game is using HT or just the cores) your CPU is maxed out (50% of 12 matches number of physical cores the CPU has), so the stuttering will be noticeable and the boost will not be as high (do bear in mind that Intel's TurboBoost is time limited, thread limited and TDP/thermal limited).
On laptops, it is also worth checking and re-plugging power cord and checking which Power Plan is used by the system.
If you want to test the behaviour of your CPU and its boost, Prime95 has nice torture tests in which you can select amount of threads as you like.
you could try to increase some graphics settings to force more load to the gpu, it's only utilized with ~50%. Easiest way is to increase the resolution scale, try increasing it in 10% steps until your gpu is almost fully utilized)
that may reduce cpu load and give you some more headroom for the boost, depends on the cooling solution for cpu/gpu I guess. but more heat from the gpu may also affect the cpu cooling if the cooler is shared for both components...
use a tool like hwinfo ( https://www.hwinfo.com/ ) to see if thermal throtteling is active.
maybe lifting up the back of the laptop gives it some more air to breathe and lowers temps.
if thermal throtteling is active, you may think about buying a dedicated laptop cooler or repasting the cpu/gpu cooler