It might not be as bad as could be feared, specifically regarding the CPU at idle anyway. A lot of modern CPUs support turning cores individually on/off (or at least into very low power sleep states) as needed and if the OS scheduler is bright enough then taking advantage of this can be a lot more power efficient than trying to fiddle around with variable clock rates. There might be a performance hit for single one-thread tasks of course as per-core performance might be low, but at times when you care (while in active use, interacting with an app) there will be at least three distinct tasks going on: core function management, display management, and at least one user task. While the watch is idle there will just be one active task most of the time so only one core needs to be powered up (of none most of the time, with device management tasks and user apps that respond to events/notifications only waking up on interrupt).
Having said that, having to charge it at least once most days is my only major complaint with the MS Band that I wear most of the time. It would be interesting to see how well this manages in that regard.
