Individual transistors will break, the cpu will continue to work, but it will run slower. That's probably why your old laptops are "too slow for the modern web

Individual transistors will break, the cpu will continue to work, but it will run slower. That's probably why your old laptops are "too slow for the modern web". " Some circuits and buses are protected by error correcting codes (ECC). If a transistor becomes marginal—meaning that it works most of the time, but occasionally fails—and thus only causes occasional glitches in those buses, then the processor may continue to function. However, it will likely trigger ECC correction events and suffer performance degradation. These ECC events are a sign it’s time to replace the processor. If that transistor fails completely, the processor may still limp along, but it will not be running at full spec if it does. "