They aren't seperate pools, its the same hardware and bus thats shared between the two its just slower if you access part of it so the CPU accessing memory is likely to be slower than on the PS5 and have a larger impact on bandwidth overall as well. This isn't a optimisation this is a trade off to save on having to spend on 20GB of GDDR6.
Now that I think about it
I am very curious how much is reserved for the OS. MS will want the fast area to be a priority to not affect the overall bandwidth too much. Assuming 2 GB are reserved for the OS I presume that will be on the fast area? That means there are 8GBs left on the fast area and 6GBs on the slower area for games.
A game that requires 9GBs will surely access the slower area too, reducing the bandwidth further then?