Some has even created a RAID between a motherboard-mounted NVMe and one in an external enclosure! - that blows my mind, and I have no idea 'how'/what would be efficiently controlling the overall data read-writes, so the whole thing is balanced for speed/volume/interface!!! So 'ultimate speed'sounds like it therefore comes from 'any' RAID-array of NVMe drives, in an external enclosure over Thunderbolt 3. I think potentially, the 'external enclosure with the PCIe/NVMe drive Thunderbolt-3, may even be comparable to a motherboard-seated NVMe? As, what I understand overall is that the read-writes on 'whatever SSD, on any interface', maybe the more limiting factor, than any specific interface. I think this setup would be fast than anything bootable that incorporated a replacement SATA SSD for the SATA HDD, as the limitation is SATA. Whereas, if swapping out both drives, then clearly there is 'nothing left' to work with, that the Mac would now work with.Ī curved-ball suggestion seems to be running a new PCIe/NVMe drive in an external enclosure over Thunderbolt 3 as one's bootable macOS drive, potentially leaving the existing SSD & HDD where they are (but decoupled as a Fusion Drive), to just become 'some storage'. I have read that is you are only replacing one drive (or want to dismantle the Fusion drive but keep the same hardware), then there can be issues with the Mac trying to 're-fuse' (boom boom), these peices of hardware - so the manual decoupling of the Fusion drive is important. Then the iMac can be opened up to replace the HDD with an SSD most commonly, sometimes the PCIe/NVMe. I've read that it's possible to disconnect the Fusion Drive either by booting from a USB media, or Time-Machine backup, or by being fast at POST and, invoking the Recovery Console, finding the exact designation of both disks, and then using this and following the command-line syntax to 'break' the Fusion apart. I have read a lot on these and other forums about disconnecting Fusion-drives, to replace the PCIe/NVMe and/or HDD with new SSDs (PCIe/NVMe + SATA). My iMac 19,1 is painfully-slow, due to it's Fusion-Drive.

Imac fusion drive vs ssd benchmarks