yes I'm reading here and there that Marshmallow takes anywhere from a slight to a significant performance hit when using any SD card as "internal storage", depends on the device though, top-of-the-line, flagship devices + expensive ultra fast SD cards won't cause issues, but most of us wanting to use that feature own devices with 16 or 8 GB of internal storage and generally with a hardware that isn't state-of-the-art regarding SD card transfer and write speeds, it's a nice idea though for future Android versions when hardware comes down in price and every device can afford ultra-fast access to SD cards.