Yep. And the only one to blame is me.
It must be using the more recent rk3588_bl31_v1.26.elf some time ago that led to doubled memory latency. I had the time to rebuild an Armbian Focal image just now from scratch (that uses rk3588_bl31_v1.25.elf just like Radxa’s own images) and results are the same (within Geekbench’s results variation) as Radxa’s.