It's several things. Desktops dissipate more heat, so sustained clocks are possible. Max Turbo is theoretical in most laptops. A decent example of this phenomenon is the m5 vs m7. Though the much is theoretical faster by double digits, in practice the clocks that those systems can sustain are identical (so save $100 by getting the m5).
Let's compare a 8550u and 8700k. The desktop has a base clock of 3.7 instead of 1.8. it has 12mb cache instead of 8mb. Twice the bus speed. Faster supported memory and more bandwidth. Desktop also has 16 pcie Lanes vs 4 (both can access more via the chipset, but then there's that bus bandwidth issue). Instruction and feature support is almost identical (except some things like vpro).
Sustained clocks, bandwidth, and more cache make a huge difference.
Very true, trying to evaluate laptop CPU performance is highly deceiving. Unlike desktop parts the actual performance depends completely on the particular laptop's cooling capability, which is impossible to know from looking at the spec. And often that cooling is terrible.
That's why desktop high end CPUs have such a large gap in performance in desktop vs. mobile. The gap seems small if you just look at the specs, but is much larger in practice.
Let's compare a 8550u and 8700k. The desktop has a base clock of 3.7 instead of 1.8. it has 12mb cache instead of 8mb. Twice the bus speed. Faster supported memory and more bandwidth. Desktop also has 16 pcie Lanes vs 4 (both can access more via the chipset, but then there's that bus bandwidth issue). Instruction and feature support is almost identical (except some things like vpro).
Sustained clocks, bandwidth, and more cache make a huge difference.