I've been experimenting a bit and it seems that the high CPU usage occurs even when instruments are not actually in use?
As mentioned at the start, I have 7 instances of the Synchron Player in Vienna Ensemble (strings, solo strings, woodwind, sax, brass, percussion, choir), each one loaded up with around 16 instruments. That way I can use the same Sibelius playback configuration for almost any score irrespective of the instrumenation - I produce around 40 scores a year, so saving separate playback configs for each score would be unmanageable.
This has worked fine for years with VI. But it doesn't work with the Synchron SE - the CPU usage is too high and I get clicks and pops in the audio, as noted above.
However, when I set up a new playback configuration with only the instruments I actually need for this score, it worked fine, with CPU running at about 25%-40%. That seems a bit odd ... why would it need to do all that "convoluting" for instruments that aren't even in the score?!
Mike