I also had high CPU utilization on two macOS machines, one being a MacBook 12" (2017) which was overwhelmed by Resilio Sync.
I am managing just two folders, but each has 1,654,405 files combined, over many sub-folders. Sync was not transferring files despite the high CPU: all files were in a synced state.
It turns out that Sync was periodically re-scanning all of the files/folders every 10 minutes! After disabling this, using the "power user preferences" option "folder_rescan_interval = 0", the high CPU usage disappeared completely on both machines.
Sync was still able to detect changes to each folder. I suspect they are using the macOS FSEvents APIs where the OS tells Sync which files have changed without polling.
I would love to know why the folder_rescan_interval isn't set to 0 on macOS by default? (And Windows, where I'm guessing the equivalent Windows Change Journals are being used?).