For SWAN_SYNC to work you need to restart the computer, after you create it. You also need to specify in Boinc Manager to leave at least 1 CPU core/thread free to benefit, but leaving more than one free improves things even more. On a 4core/8thread system there is usually no benefit in using less than 50% of the CPU (4threads) and not much difference between using 4 and 5 threads. I normally only use between 5 and 7 threads 62.5% to 87.5% of the CPU.

Ahh, OK. I had restarted the client and figured that was enough. Strange that the performance changed at that time if Swan_Sync hadn't kicked in at all. Perhaps it was just coincidence and the tasks had a natural "smooth spot" at that point.

After the reboot the tasks are indeed using a full HT thread each.