@SourceCreations: If you aren't already very familiar with it, it can get confusing. Both motherboards support the same connections that come directly from the CPU rather than via the chipset. Those are:
The memory interface. (Actually the B550 board supports higher memory speeds at DDR4 5100(O.C) and it supports ECC memory.)
They both have one PCIe 4.0 x 16 slot for the graphics card.
They both have one PCIe 4.0 x 4 M.2 slot for the primary NVMe SSD.
They both have four USB 3.2 gen 2 ports (10Gb/s each) directly from the CPU.
This is the main part of how motherboard hardware affects the PC speed.
Then we get to the chipset itself. There isn't a lot of stuff left on an ITX motherboard that connects via the chipset. It is the secondary M.2 slot (supported on Asus ITX boards), extra USB ports, SATA ports, ethernet, WiFi, sound and other peripherals. The speed difference is that the X570 chipset is fed by a PCIe 4.0 x 4 and the B550 chipset is fed by PCIe 3.0 x 4. As such, there is twice the speed headroom available for that small collection of chipset fed ports and devices I listed. In practice, using enough of those at once to see that headroom is not easy to do. Especially since the most data hungry devices are already connected directly to the CPU instead.
On an ATX board the chipset matters more because it supports all the other PCIe slots and usually more I/O ports as well.
Here is a helpful quick comparison including these ITX boards: Optimum Tech: B550 ITX vs. B450, X570 - Worth it?
I have both an Asus X570-I and an Asus B550-I motherboard amongst my gear. The Asus B550-I has an internal header for the case USB C port, which the Asus X570-I lacks. Both have an internal header for 2 x case USB 3.1 Gen 1 ports. The Asus B550-I has 2.5Gb/s ethernet instead of the 1Gb/s on the older Asus X570-I. The Asus B550-I has an audio type C port and a BIOS flashback/general USB 2.0 port (plus the BIOS flashback button) instead of four USB 3.2 gen 1 ports (5Gb/s) on the Asus X570-I. (This doesn't mention all the I/O.) Coupled with around 1000 kr cost savings, buying a new Asus B550-I for my newest build was a no-brainer for me.