Also to clarify, the B550I's still have 2x M.2 slots, the difference is one slot (front) is PCI-E gen 4 x4 (~5000MB/s limit), and the rear M.2 is PCI-E gen 3 x4 (~3500MB/s limit) where the X570I has 2x PCI-E gen 4 x4 (1 front, 1 rear). The extra 1500 MB/s is pretty significant on the second M.2 if you have a gen 4 supported NVME SSD, however it will run much more warm and could thermal throttle in the rear port anyway on the X570I, possibly making it as slow or shortening the life of the drive. I would probably limit my rear M.2 to gen 3 in the BIOS anyway.
Another thing to consider is in gaming currently there isn't a big difference from Gen3 to Gen4 in load times, though there is a possibility that could change in the future with DirectIO support, but we aren't there yet.