I guess I'm hitting controller limits!

[sweh]

% hdparm -t /dev/sda /dev/sdg

/dev/sda:
 Timing buffered disk reads: 708 MB in  3.00 seconds = 235.98 MB/sec

/dev/sdg:
 Timing buffered disk reads: 1064 MB in  3.00 seconds = 354.67 MB/sec

These are both SSDs (Crucial_CT512MX100). sda is plugged into the motherboard and reports

ata1: SATA link up 3.0 Gbps (SStatus 123 SControl 300)

sdg is plugged into a PCI adapter and reports:

ata7: SATA link up 6.0 Gbps (SStatus 133 SControl 300)

I have these two devices in a RAID1, so the resulting speed is limited.

Spinning rust returns around 125MB/s so it's still almost twice as fast, but I guess I've now gone from rust speed limits to controller speed limits!