eMMC does have a wider bus and dual edge clocking so it's not a straight comparison. I think read might be fundamentally faster on eMMC but writing mostly depends on the NAND chips used inside.
UHS-I did add a dual-edge mode at 50MHz, but SD never got up to the 8 bits MMC supports (also at about 50 MHz, so UHS-I should be roughly half the speed of high-end MMC). UHS-II went to LVDS instead, which despite using fewer lanes can support much higher clock rates, so it’s not immediately obvious which is theoretically faster with newer versions. I need to get to work or I’d do the math.
UHS-3 doubles the LVDS speed, and SD Express turned them into a PCIe lane. I haven’t kept up to see what MMC standards have been added lately, if any, but my impression is that it has been stagnant for a while.
UHS-3 doubles the LVDS speed, and SD Express turned them into a PCIe lane. I haven’t kept up to see what MMC standards have been added lately, if any, but my impression is that it has been stagnant for a while.