It's write speed; sorry about that, I should probably be more specific! All the benchmarks I'm using are from this Wiki page (http://www.pidramble.com/wiki/benchmarks/microsd-cards) — though I need to update that page to include some more USB/SATA/NVMe performance benchmarks since I'm now using a lot more drives with the Pi 4 and CM4 (and skipping microSD entirely for most builds).