
I think the reason is that they removed gpu decode parts in cpu decode mode (yes, it sounds weird), thus the slowdown in speedtest app. 12K ursa files were previously partially decoded with gpu because decode wasn't fully optimized for cpu instruction sets yet. This in turn created problems in some applications due to gpu memory getting filled even in cpu mode.
In BRAW SDK release notes, it is mentioned that "Added optimised CPU decoding for clips captured by Blackmagic URSA Mini Pro 12K."