Greetings,
You will need to look closely at how the manufacturer implemented support. They are many factors that can affect disk performance. Chipset type, CPU type and number of PCIe lanes, etc are examples of what might affect bus speeds of interfaces on a system.
This is why you'll see performance tables in a motherboard's user guide. Support and performance can be determined by the type of CPU, and the number and types of disks being used. Priority is often given to PCIe slots for GPU's, but today one good card can often perform at x16 nearly as well as 2 in SLI at x8. I usually base the configuration on what the system will be used for primarily.
Gaming and highest frame rates. Photo editing and video production, etc. 2D/3D modeling. I do a lot of photo and video on my systems. I have a few MACs too, but don't really care for them.
If you read the fine print in your user guide, you can see the difference performance levels and how resources are allocated if using a 10th vs. 11th gen CPU.
Here is an example:
Note:
The M2_CPU (upper) slot supports PCIe 4.0 spec and x4, x2
The M2A_SB (lower) supports PCIe 3.0 Spec and x4, x2
Now see the fine print:
(Note) Supported by 11th Generation processors only.
So, the top slot will not support PCIe 4.0 spec unless you are using an 11th gen CPU. Its also noted in the Slot naming convention used. Clearly the M2_CPU is interfacing the "CPU" directly, whereas the M2A_SB is interfacing the bus through the integrated "SB" south bridge of the Z590.
Be careful choosing PCIe expansion cards for adding additional M.2 drives. Even if the card is x4 and you insert it into a x4 slot, resource availability or chipset compatibility might not allow it to run at full speed, and it might cut the speeds of an adjacent slot in half. This is because the slots can share resources.
Hope this helps
