I think these SD-to-IDE adapters (which are actually an SD-to-CF adapter and a CF-to-IDE adapter glued together) are limited to about 25 MB/s throughput, but you're still benefitting from low access times.
If one is running these on anything earlier than 2000 with Mac products, this should be more than adequate, as 2000 marked when Apple’s products moved to the ATA-4 IDE standard, with peak bandwidths reaching 25MB/s (and UDMA/33, or 33MB/s, on the late ATA-4 products, such as the clamshell Rev. C iBook).