For me passing a raw ssd device has worked best when using the virtio-scsi device model, always giving near-native performance for me. See my setup notes and performance numbers here: GPU passthrough straightforward with Coffee Lake + some "duel booting" notes and issues
Also, are you using NPT=1 and are you sure that the software you are using includes the fixes for the NPT bug? I/O is typically something that is affected negatively by NPT=0, though IME virtio-scsi offsets a lot of that problem too (this shouldn’t be a problem anymore in either case, given a new enough linux kernel… or maybe you still need to patch, I am not sure whether the fix has made it into the kernel).