@msfujino thanks for the idea, but it’s not updating the display at all with your hardware spi - code. Instead, last image from last soft-SPI call is shown.
I tried with different clock frequencies already with no big changes.
By “slow” refresh I’m talking about ca. 3Hz (!) for a rectangle and a random number while no other code is executed but the display refresh.
Sorry: it’s running perfectly fine with your HW-SPI suggestion, when NOT using mbed. With mbed = no updates whatsoever to the screen.