A benchmark between the iPad Air 4 (A14) and the 2021 iPad Pro (M1) showed the write speed of the internal storage was essentially 3x faster. 589MBps vs 1532MBps. I think in the year since that benchmark, updates to iPadOS have allowed the iPad Pro M1 to achieve higher speeds, the same benchmark giving a result of 2169MBps.
Not only do you have a much faster SSD, but you have more RAM. An older iPad with Stage Manager might need to use swap more because it has 4GB/6GB of RAM in the first place, and then the swap itself is much slower. That can compound into far worse performance where the thing you need to use more is slower.
I can't seem to find how fast the 2020 iPad Pro storage is though. Looking through some benchmarks for the 2018 iPad Pro shows that it is slower than the iPad Air 4, somewhere in the 300MBps range - and that iPad Pro only had 4GB RAM in the base model.
I'm not saying the above is evidence that stage manager couldn't run, but it's enough to reconsider that Apple's explanation MAY be truthful. And I'm not pro-Apple on this. I for one think they skimped on RAM amount and storage quality in the 2018 and 2020 iPad Pros which is inexcusable considering the pro prices they charged for it and the fact they probably knew pro features requiring better hardware were on the horizon.
It should be noted that capacity makes a big difference in sequential throughput. Performance may also be affected by stuff under the hood like RAM caching, pseudo-SLC caching, etc. These can be the reasons why the benchmarks aren’t always consistent. Also, sequential performance isn’t the be all and end all. I’d argue IOPS and random read/write are more important for an OS drive.
I don’t think Apple skimped on storage
quality on the 2018/2020 Pros. The SSD controller is
part of the Apple chipset and they’ve been improving that generation after generation. The A12X storage performance is significantly better compared to A10X and A9X. Iirc, A9 is when they added the SSD controller to the chipset post-Anobit acquisition. Apple does have a history of skimping on base storage
quantity.
Mind, Apple’s always been stingy with RAM (at least on iPhones and iPads). The 8/16GB on the 2021 Pro came as a total surprise to me. I thought we’d be stuck with 6GB for a few more years (4GB was pretty much top RAM from 2015 to 2019, excluding the non-standard 6GB/1TB).
At least for my usage (mostly Safari, Goodreader, Excel, Dropbox, Google apps):
4GB: Okay for split view+slide over-type multitasking up to iOS 12. Sadly, I’ve had
active split view apps crashing due to lack of memory after updating to iPadOS 13.
6GB: Okay for split view+slide over-type multitasking on iPadOS 13+ but doesn’t appear to have much headroom.
16GB: Plenty of headroom.
Here are benchmarks for a 256GB Air 4 and 1TB 2020 Pro.