SDMMC does not work at 40 MHz

[JPMJPM]

Hi, I am testing examples/SDMMC/SDMMC_Test with those results ("ESP32 Dev Module" 2.0.6) :

1) With SD_MMC.begin() -> Read 1 MByte in 191 mS and Write 1 MByte in 415 mS
Card clock is 20 MHz measured with Oscilloscope

2) With SD_MMC.begin("/sdcard", false, false, 40000, 5) -> Read 1 MByte in 191 mS and Write 1 MByte in 423 mS
Card clock is 40 MHz measured with Oscilloscope

Sure I'm doing something wrong but I don't know what.
Please help, thanks.