How many files InSync is monitoring on that particular share/storage?
From my personal experience, btrfs has very high overhead on small files on single disk systems, so I’m not using it on any single/personal drive which requires high performance. LUKS also adds its own overhead to the filesystem operations, so it might be creating a perfect storm.
Maybe you can try writing many random small files with a small script (for + dd) comes into my mind, and see whether you can reproduce the spikes that way.