Martin, many thanks for your comment.

I know woefully little about how MacOS actually works and had a vague notion about the SSD part of a Fusion Drive as being simply extremely fast static storage for much of the OS itself as well as various apps and stuff favoured by Apple as being important for "the Apple Experience".

So does MacOS prefetch from the HDD and cache stuff in SSD, somewhat like CPU prefetch caching? (It's ok, Martin, I'm not asking for a MacOS tutorial, lol.) Whatever, I'm certainly prepared to agree that the Fusion Drive may seriously complicate the issue here.

When I get time I'll try moving a big VSL library back into one of my bootable clone backup hard disk drives (LaCie D2 Thunderbolt with 7200rpm Barracuda delivering up to 220 MB/s best read speed), then boot the iMac from that drive and see what Synchron's speedometer says. And while booted in that HDD I could also try switching the Synchron Player's library path back to the external T7 SSD and check Synchron's speedometer readings again. I'll get back to you with test results asap.

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.

@Macker said:

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.

When you load a library into RAM, a lot of the data will be read sequentially from the SSD. The Synchron player tests non-sequential 64k reads, which have a lower throughput of data.

Of course that doesn't explain the difference between Windows and Mac versions. 

John, I'm well aware of implications of the distinction between 'sequential reading' and 'random access' from bulk data storage, thanks all the same. (The point I was making to Martin was somewhat rhetorical.) My point is the Synchron speedometer seems counterintuitive. If it showed the result of actual reading from storage (say like in the Mac's Activity Monitor) then at least users could get the hang of the performance of their various storage drives under various different conditions. After all, some computer-based music productions can get to be pretty hairy, wing-and-a-prayer, seat-of-the-pants stuff, and in those circumstances it can be very, very useful to have some idea of what's actually going on under the bonnet.

But no, even when loading a big, nicely-ordered factory preset it seems the Synchron speedometer ignores what's actually going on and stubbornly reports the result of its worst-case testing. That's what I call counterintuitive. It's worse than trying to mix with only PPMs on the meter bridge. Well, theory and practice often tend to diverge quite a bit in the real world.

It makes sense to me that it works the way it does, as it's meant to indicate the drive's ability to keep up with a demanding workload. 

What I don't understand is how it's different on Windows and MacOS. Maybe that has nothing to do with VSL.

PS I'd like to be able to see the actual throughput in the MacOS activity monitor.