mSATA → IDE → USB 3.0 (new driver)
Updating the VIA USB 3.0 driver to something more recent than what came with the card fixed the high CPU usage and gave a nice performance boost as well. The peak speed is even a little faster than what the SATA SSD in Nyx managed, though that is the system drive and so is hardly idle during tests.
mSATA → SATA → eSATA (new driver)
The same can't quite be said for the eSATA link. To be fair it is faster, but still much below what should be possible. Poking around with smartmontools suggests that this is because the drive is only running at SATA-I despite both it and the controller supporting at least SATA-II.
|Cabling||Controller||Max (MB/s)||Average (MB/s)|
|mSATA → IDE → USB 2.0||Intel||33.3||32.8|
|mSATA → IDE → USB 3.0||VIA||81.7||79.1||(through hub)|
|mSATA → IDE||Intel||82.1||81.1||(as installed in Aether)|
|mSATA → SATA → USB 2.0||Intel||33.3||32.9|
|mSATA → SATA → USB 2.0||VIA||26.2||25.1|
|mSATA → SATA → USB 3.0||VIA||138.4||129.5|
|mSATA → SATA → USB 2.0||VIA||25.4||24.7||(through hub)|
|mSATA → SATA → USB 3.0||VIA||135.4||129.4||(through hub)|
|mSATA → SATA → USB 3.0||VIA||224.6||203.3||(through hub) (driver updated to 5.20A)|
|mSATA → SATA → eSATA||JMicron||64.2||62.9|
|mSATA → SATA → eSATA||JMicron||117.5||112.8||(driver updated to 22.214.171.124 eSATA)|
|Plextor 256GB M5S SATA||Intel||220.5||217.3||(as installed in Nyx, limited to SATA-II)|
And with that, the puzzle is solved! Now to put the laptop back together (did I mention I was using the SSD from it)...