If I was you, I’d use the drive that’s giving you >150, those internal drives must be exceptionally slow. If it’s still bogging down on you, then at least you know it’s probably not the drive.
I don’t know why the read/write times matter so much, I’m just regurgitating what I’ve been told/have read.
Maybe someone more knowledgeable can chime in here…?
Are you sure you are using the Blackmagic 9.6.1 driver??
You also said that you’ve tried MPE hardware – you’re card is NOT on the supported list. Did you add it to the list to get it to work? This may not be the optimal thing to do with your unsupported card.