Thank you for the fast response and update!
As I'm a application engineer myself I tried both suplied versions and can confirm that on my mac OS Siera PB5 the cpu usage is back to normal. I'm now using Resilio Sync 2.4.0(640), I have the feeling that that version uses even less CPU than 2.3.9. The startup usage stops faster, also when idle cpu usage is less than 1%. So, I'm happy