Martin, here are the new test results I promised.
After booting iMac from external LaCie-enclosed Barracuda 7200.14 cloned High Sierra backup HDD, formatted as Mac OS (Journaled), connected via Apple T3/T2 Thunderbolt adapter to Thunderbolt 3 port, I ran 2 tests.
Test setup same as posted above - Clarett 2pre Thunderbolt audio buffer: 256; LPX 10.4.8: 1 instrument; Synchron Player: 4 threads loading; 4096 preload; force all slots enabled.
VSL preset selected:
15 SYzd Dimension Strings/Dimension Strings | Full/01 1st Violins/1st Violins full - All players (2.64 GB)
• Test 1: Synchron library path: external boot drive LaCie/Barracuda.
Time to load: 21 mins 38 secs (including initial cursor beachball for 15 secs). (Roughly 2x slower than path from iMac Fusion Drive.)
Synchron speedometer after preset loaded: 50.9 MB/s (About 4x slower than path from iMac Fusion Drive.)
• Test 2: after changing Synchron Library path to external T7 SSD connected via thunderbolt 3 port, preload 4096, then rebooting again from the external Barracuda.
Time to load: 53 secs (including initial cursor beachball for 13 secs)
Synchron speedometer after preset loaded: 200.5 MB/s
So ok, the case is altered. In the previous tests it appears my Fusion Drive made substantial differences to loading time and Synchron speedometer readout in the case of Fusion Drive library path. I hadn't understood that was happening, having assumed that only the iMac's internal HDD was involved in that path.
However, I'm still extremely puzzled by @johnstaf's Synchron speedometer showing 600 MB/s for his external T5 streamed into a Windows machine.
Moreover, I don't know how to account for the fact that Synchron speedometer showed only a 4x increase in speed when using my T7 instead of the Barracuda, yet the loading times imply a 24x increase in speed for T7 versus Barracuda.