Same problem here.
I am running the Synchron Player for the first time, stand alone, with the Hello Free Instruments. All instruments seem to work as expected except for the Fujara Flute, which puts heavy load on the CPU.
I have an RME HDSPe Asio Pro internal soundcard. Win10, AMD Ryzen 5000.
Playing one tone only, there is a proportional relationship between the RME ASIO driver buffer size and the CPU load, as follows:
Buffer Size/CPU Load (%): 32/100(overload, no sound), 64/63, 128/38, 256/19, 512/10, 1024/5
Another interesting observation relates to which microphones in the mix that are active.
Having any or all of the following microphones active gives a very low load (eg 2-3% at buffer size 64): Main, Main-C, Surround, High, High-Sur.
Only the Mid and Close microphones, when active separately or together, contributes to the high load (61% at buffer size 64)!
Could that information get you any closer to finding a solution?