Computers have fans. Fans cool down the hardware. Macs have fan management firmware. An error in the firmware caused the fans to not ramp up as they are supposed to when things get too hot. This caused the CPU to throttle under load.
Keep in mind that the default setting for nearly all macs is to not ramp up the fan until the CPU is redlining. For example, by default my 2009 Mac Mini does not speed up its fan until the CPU gets over 85° or so. At which point it turns the fan up all the way so it sounds like an angry jet engine. All I have to do is manually adjust the lowest fan speed from “completely inaudible” to “low murmur that can be heard in a quiet room” and the idle temperature falls from 50° to 35°, with the load temperature never exceeding 60° instead of getting up into 80-90° territory (ambient room temperature kept at 25 or 26, and yes we use centigrade here in Canada, deal).
Someone’s cat walked across the keyboard and entered incorrect values into the table of default speeds for a given temperature in the fan control firmware.
