HAMEG HM512 = Two channels, 0-15 MHz (-3 dB)
Found this picture at
https://www.radiomuseum.org (museum LOL)
The SPI might be clocking at 10mhz and the scope is 15mhz.
The falling edges are slow and the rising edges are fast. Strange it is usually the other way around. For TTL type signals. Something is slow! With out knowing the SPI frequency I can't say what is slow.
1) Maybe the SPI is good and this is the scope being slow. BUT Then the going up and the going down speed would be the same.
2) Maybe the scope is OK for this speed and the SPI is too slow. Maybe too much capacitance on the bus. (maybe 20mhz)
Is there a way to slow down the SPI clock? Can you change the code?