I think a lot of people are getting worked up over something that should be clear when comparing the A12Z and the M1:
According to cpu-monkey.com, the A12Z's P-cores run at 2.49 GHz, while those of the M1 run at 3.20 GHz. That's almost a 30% speed increase, not counting improvements to IPC (instructions-per-clock), L2 cache, process improvements (5nm on M1 vs 7nm on A12Z), or RAM bandwidth between those CPU generations.
Comparing the Geekbench scores for the
12.9" M1 iPad Pro vs the
12.9" A12Z iPad Pro shows that the M1 has, in fact, a 52.2% single-core score increase and a 54.7% multi-core score increase. The M1 is
significantly faster, at least in this benchmark.
These speed increases, plus the larger amounts of RAM in the M1 iPad Pros, are likely what provides the additional power to run Stage Manager, in addition to improved disk I/O speeds for (where available) virtual memory swap.
It's also worth mentioning that, unlike macOS, iPadOS has no notion of a beachball cursor or app loading indicator (i.e. apps don't bounce in the Dock when launching), so apps need to be very responsive and the bar for performance is much higher than it is on the Mac. Also, unlike the M1 MacBooks, which all have 60 Hz screens, all iPad Pros run at 120 Hz, which increases the performance requirements further.
Apple could work on Stage Manager to make it performant on older iPad Pro models, but IMO that effort is probably better spent on making Stage Manager better and smoothening its rough edges.
EDIT: Regarding the A12Z DTK, it had 16 GB of RAM as opposed to the 4 or 6 GB in the 2018/2020 iPad Pros. In addition, many units stopped working a few months after being given out to developers due to a hardware issue - something that Apple most likely does not want to happen again with older iPad Pros.
It's also worth noting that M1 Macs initially had issues with some external displays that were later fixed with a software update. If this was the situation with M1, there are likely more issues with A12Z that Apple does not want to deal with.