A lot. Fastest microSD through UHS-I interface tops out around 100 MB/sec. NVMe SSD tops out at 450 MB/sec at Gen 2 speed, and 900 MB/sec at (technically unsupported, but working) Gen 3 speed.
I would also think that random read/write performance (throughput/IOPS and maybe latency too? just a hunch...) on SSDs will be miles ahead of microSD and likely represents impacts on real-world/OS scenarios much more than sequential numbers
So is the UHS-I support hardware bound?
Or could we get those speeds on Raspberry 4?
Hardware; Pi 4 only used SDR54 or something like that, half the data rate as Pi 5 microSD interface.