ok, i have some news:
after activating this option (red marked):
(i dont know what it means or what it does. the first option turns on the disk cache, the second -> ? maybe RAM caching, but this wouldn't speed up the raid itself i think.)
the results look like this:
partition C (system partition, standard cluster size):
partition D (data partition, 32k cluster size):
why 32k is so slow and 64k so fast?