I tried with OPL BDM 4.8.1: and the stuttering and freezing problems seem to be gone. Performance seems pretty good. There is a weird issue where I can't seem to switch to MX4SIO on OPL if I have a USB thumb drive inserted.
Not sure if you still need test logs, but here's one I gathered from a SanDisk 128GB Extreme PRO, model: SDSQXCD-128G-GN6MA
It seems to load games ok, but video playback is extremely choppy and it deadlocks sometimes. I might try the latest version
MX4SIO: R:0xff
MX4SIO: W:0x4d, R:0xff...