What you told is probably an important reason. But another one is that modern x86 cores clock down if there is load on multiple cores (to prevent heating issues

What you told is probably an important reason. But another one is that modern x86 cores clock down if there is load on multiple cores (to prevent heating issues) or even clock up for a short time (e.g. Intel Turbo Boost) if only a single core gets a load peak without much load on other cores. In practice also often insufficient cooling is a reason why the computer gets "slower/sluggish" if there is a high load in particular on multiple cores. The cooling of practically every laptop available is in my opinion undersized (to save space) and if dust gets into the fan (it will gradually) this will get even worse. Luckily most gamers ("PC Master Race") know how important a sufficiently dimensioned cooling is for their self-built PCs.