In my (anecdotal) Dell laptops have been crap at heat management. My current laptop (about 4 years old at this point) had the problem you described, except once

In my (anecdotal) Dell laptops have been crap at heat management. My current laptop (about 4 years old at this point) had the problem you described, except once the CPU throttling turned on, the BIOS was extremely conservative in turning it off, to the point where I would often reboot to get it running at speed again. Eventually they released a BIOS update that fixed the issue, (and started throttling earlier to avoid dropping to .8ghz) Point being, when buying a laptop, always ask about the thermal.