Since odrive is a bidirectional sync engine that tries to achieve near real-time remote<->local consistency, it ends up doing a fair amount of work while not actively uploading and download stuff. It has to make sure that everything stays in sync, at all times.
This means the more stuff you have that odrive needs to worry about (basically the number of files and folders), the more work it has to do, in general. Since the Pis are not powerhouse boxes, this background “idle” activity could manifest as high CPU if there is a fair amount of data to monitor.
If you don’t require that type of “vigilant” behavior, you can shutdown the agent until you need it, in which case odrive scans for changes on startup.
You can also remove items from scope by unlinking storage or unsync certain folders, which can reduce overhead.