SD cards as present on Raspberry Pi, even the Pi 4, don't go beyond ~60MB/s. Older Pis get around ~20MB/s. It's the worst bottleneck of the system. Where do you get that 1 GB/s figure from?
Bear in mind that SD Express is SD only in name and otherwise it is just an NVMe in SD form-factor. And how much UHS-II is still an SD is somewhat questionable.
https://en.m.wikipedia.org/wiki/SD_card
SD Express can do up to 985 MB/s.
SD UHS-II @300MB/s would be plenty for most SBCs.