Sync can be a heavy process because it is constantly trying to keep both sides in sync, scanning and monitoring both the remote and local locations. If you have sprawling structures/lots of objects to track it can create some heat.
The backup feature in the agent is a really early prototype. There are folks that use it, but it isn’t something we ever released as a finished product.
Reducing the amount of work that odrive has to do can reduce CPU. This can include partitioning data out so odrive only has to scan a subset, or unsyncing folders to collapse the structure.
You could also try something like nice/renice on the agent process to reduce its priority. This may not prevent 100% CPU utilization, but it will ensure other applications get priority if you have other apps battling for CPU.
There are other utilities that can try to keep a process under a CPU ceiling, like cpulimit.