Refer to ERR007926 in IMX6DQCE(Rev.6).
There is below description in SDMMC boot Workaround.
"SD/MMC: Choose an SD/MMC card for which the Nac parameter is to be specified less than
50 ms and its initialization time is less than 500 ms."
I think that Nac parameter is TAAC + NSAC from SD Specification.
But, I didn't find the parameter of "initialization time".
Could you tell me the parameter?