They also use another CPU than any other phone is using. I’m sure it is the main reason for the temperature. And nothing easier than fixing temperature: reducing CPU cycle speed to much less power. But I’m sure that’s also nothing people want.
Another way would be to improve performance of applications, but that is a job of app-devs, not of Purism. But let’s be clear: the heat is nothing that will ever be “fixed” with current version of L5. Therefor we would need a better CPU that can also be used free of proprietary blobs. Maybe L5v2 or later could solve this.
Btw this is no design issue. There was just no better possible solution.