I use IDE2SAT2 from https://www.startech.com/en-ca/hdd/adapters?filter_hddadaptype=adapter%20converterI have some thoughts about doing the same to an old 2nd gen Bondi Blue iMac that I have. What kind of adapter is needed to get one of those to take an SSD drive?
which says it can do up to 133 MB/s (ATA-7) which is the max speed of a PCI slot.
I don't know if it can fit in an iMac.
Everymac says "This model supports a single internal 3.5" EIDE (ATA-3) hard drive. It does not support hard drives larger than 128 GB."
https://everymac.com/systems/apple/imac/specs/imac_ab.html
Wikipedia says ATA-3 supports at least 16.7 MB/s.
https://en.wikipedia.org/wiki/Parallel_ATA
Some other source say ATA-3 supports up to 33 MB/s.
https://superuser.com/questions/408623/what-is-the-maximum-throughput-of-an-ide-bus-ata-3
So I don't know what speed you should expect from the iMac.