I’m looking at the SK4 CE DTB. The mmc controller uses an 8bit bus at 200Mhz (1600Mbps), HS400 doubles that (DDR) to 3200Mbps = 400MBs maximum possible
What happens when you perform an actual eMMC write test
dd if=/dev/zero of=/data/local/tmp/testfile bs=1M count=1024; sync