Decoding the SACD is mostly single-threaded, which is why you're seeing 60% usage of a dual-core. That likely means it's running one core at full speed, and it's not quite enough.
An 817 JRMark may just not be able to handle real-time DST decompression across multiple channels with resampling. To compare, a medium range computer scores around 2000 and a fast computer will score over 4000.
I got these results === Running Benchmarks (please do not interrupt) ===
Running 'Math' benchmark...
Single-threaded integer math... 5.090 seconds
Single-threaded floating point math... 3.013 seconds
Multi-threaded integer math... 2.611 seconds
Multi-threaded mixed math... 1.580 seconds
Score: 1545
Running 'Image' benchmark...
Image creation / destruction... 1.856 seconds
Flood filling... 1.017 seconds
Direct copying... 2.100 seconds
Small renders... 2.404 seconds
Bilinear rendering... 1.633 seconds
Bicubic rendering... 1.531 seconds
Score: 2087
Running 'Database' benchmark...
Create database... 0.810 seconds
Populate database... 3.561 seconds
Save database... 1.634 seconds
Reload database... 0.231 seconds
Search database... 2.358 seconds
Sort database... 1.844 seconds
Group database... 1.397 seconds
Score: 1817
JRMark (version 17.0.180): 1816
Do I need a faster computer for multichannel playback form iso files? No trouble with stereo but can not play back Multi-channel without buffering.