There's always going to be a trade off between speed, practicality, and energy efficiency. But for some long-duration tasks, it's probably best to use a microcontroller that doesn't need active cooling

Commodore 64 systems were used in industrial settings for decades. Some are probably still running

If the heat and power budget is bigger, probably something like a TTL (or modern CMOS equivalent) chip based basic computer can keep things running for just as long. And still be user serviceable