Two laptops with the same processor and graphics chip can deliver noticeably different frame rates in the same game. The difference is thermal capacity, and it is the defining constraint of the format.
Power turns into heat that must leave the chassis
Every watt a component draws becomes heat, and a laptop must move that heat through a thin body using small fans and limited airflow.
Desktop cases have volume, large heatsinks and slow-turning fans to work with. A laptop has none of these and must produce equivalent cooling from a fraction of the space.
When heat is produced faster than it can be removed, temperature rises until the chip reduces its own clock speed. That reduction is throttling, and it is protective rather than faulty.
Manufacturers set a power budget per model
The same graphics chip is sold to laptop makers with a configurable power range, and each maker chooses a figure the chassis can sustain.
A thin machine may run the part at the bottom of that range, while a thicker one runs it near the top, producing a substantial performance gap between identically named products.
This is why specification lists alone are unreliable for laptops. The named component describes what is inside; the chassis determines what it is allowed to do.
Shared cooling makes the two chips compete
Many designs run heat pipes across both the processor and the graphics chip, so heat from one raises the temperature of the other.
Games load both, and the total budget is divided between them dynamically. A processor working hard therefore reduces the headroom available for graphics.
Manufacturers manage this with a combined power limit, which is why a laptop can appear to lose graphics performance in games with heavy simulation.
Behaviour changes over the first minutes
A cold machine performs well because the chassis has heat capacity to absorb before temperatures rise. Peak figures are often measured in this window.
After several minutes the system reaches equilibrium and settles at whatever the cooling can sustain indefinitely. That steady figure is what a long session actually delivers.
Short benchmark runs therefore overstate laptop performance systematically, which is why sustained testing gives a very different ranking from brief tests.
The practical levers are environmental
Raising the machine to allow airflow underneath, keeping intakes clear and running in a cooler room all raise the sustainable limit, sometimes noticeably.
Undervolting and frame rate caps work from the other direction by reducing heat production, which lets the system hold higher clocks rather than oscillating.
What none of these change is the ceiling built into the chassis. The design decided the sustained figure before the machine was assembled.