The SPI clock of XIAO nRF52840 is 8MHz when mbed is used for BSP, but can be set up to 32MHz when non mbed is used.
It depends on the library you are using, for example
tft.setSPISpeed(32000000UL);
You may also refer to the following link.
‘XIAO_nRF52840 mbed's SPI performance is only 1/3.5 of non-mbed's’