I quickly checked with the team, and the most likely explanation is that the move to the Debian Bookworm base also changed the CPU frequency governor.
On Debian Bullseye, the default governor was typically ondemand, whereas newer kernels used by Debian Bookworm generally default to schedutil. The main difference is that schedutil adjusts the CPU frequency much more dynamically based on the scheduler, while ondemand periodically samples CPU utilization and often keeps the CPU at higher frequencies for longer.
As a result, seeing the CPU frequency continuously jump between different clock speeds is expected behaviour with schedutil. This can cause the reported CPU utilization to appear slightly higher (because the CPU spends more time at lower frequencies).
So, in essence, what you’re seeing is a Homey that is managing its CPU resources more efficiently. Instead of keeping the CPU running at its maximum frequency all the time, it continuously scales the clock speed up and down to match the current workload. This reduces unnecessary power consumption and heat generation while still allowing the CPU to ramp up to full performance whenever it is needed 
Homey starts throttling at 85°C so that isn’t whats happening here. You can also double check this by going to the Homey Developer Tools → System and check if the throttle values are true or false.
