In my Acard ARS-2000SU with a CF to SATA adapter, I noticed that certain CF cards are much slower than others. Specifically, a 32 gig card will run at whatever speed the Quadra 605 it's in can run, but another card (a 64 gig SD card in an SD to CF adapter) works perfectly but much, much slower.
Some cards can only do very limited kinds of transfers, such as this 2 gig card in one of my Amigas:
wd0 at atabus0 drive 0:
wd0: drive supports 1-sector PIO transfers, LBA addressing
wd0: 1959 MB, 3982 cyl, 16 head, 63 sec, 512 bytes/sect x 4013856 sectors
I don't care about the speed since it's only in there to boot the machine, but if I were using it as my primary disk I'd not want to use a 1-sector-at-a-time card.
As a separate issue, the Acard chip is definitely not a bottleneck. On other m68k machines I get 3 MB/sec write, 5 MB/sec read via an Acard and a 500 gig hardware mirrored SATA setup, and on a PowerMac 9600 I get 22.5 MB/sec write and 46 MB/sec read on two hardware mirrored 2 TB SATA drives, also via an Acard. They're quite fast.