Ah, okay, then I'm using a different definition of throttling. Intel will publish as you say a base speed of 1.8 GHz, then a single core Turbo speed of 4.6 GHz

Ah, okay, then I'm using a different definition of throttling. Intel will publish as you say a base speed of 1.8 GHz, then a single core Turbo speed of 4.6 GHz and also an all-core Turbo speed of 4.2 GHz (numbers made up, but something like this). If sufficient cooling is available, the CPU should be able to sustain the all-core Turbo number indefinitely. If it can't, I call that throttling. It can be mild (if you go from 4.2 to 3.2) or severe (if you go from 4.2 to 1.8). A colleague has the XPS 15 (well, the Precision equivalent) and he's never seen it drop below base clock, the problem with it is that base is something ridiculously low like 1.2 GHz. If a machine drops below base freq. due to thermal issues, it has been designed ver wrongly. Our workstations with water cooling run at maximum all-core Turbo freq. for days on end. Those CPUs do exceed the specified TDP when doing so, which is fine as long as the cooler can easily dissipate that heat. And you can get water coolers that support 500W TDP, so no worries. The only guarantee Intel makes is that the processor will stay within TDP when running at the base clock. What's happening when your laptop goes briefly to 4.2 GHz is that it exceeds both the Intel-stated TDP and the cooling system TDP. Then it throttles back to 3.2 GHz, which is a little below the cooling system TDP but above the processor TDP. In a laptop like the XPS 15, the cooling system TDP is only a little higher than Intel-stated TDP. The momentary thermal headroom between what the CPU puts out at max Turbo and the cooling system TDP is provided by the heat capacity of the metal in the heatsink/heatpipe.