RP2040, Arduino, SPI1, change SPI1 clock speed


I have a working SPI1 (for SDCard) on RP2040, but the max. speed I can set is 4MHz.
The SPI1 I have defined in the SD-Card library, Sd2Card.cpp as following.

arduino::MbedSPI SPI1(SD_MISO, SD_MOSI, SD_SCK);
settings = SPISettings(312500000, MSBFIRST, SPI_MODE3);

How can I increase the SPI-Clock to 8MHz or more ?

platform = raspberrypi
board = pico
framework = arduino
upload_protocol = picotool
upload_port = E:\
lib_deps = arduino-libraries/SD@^1.2.4

Thank you