What actually triggered me to do that was to be able to test repeatably my other open source project in rust were the compile target directory can go to hundreds of gigabytes of files and at the same time I need to benchmark it on large linear slabs we do create for memory mapped files and I definitely noted the impact. I can only guess but non-optimal extent length create a repeated round trip to the metadata in main memory instead of "linear reads" (as consecutive logical numbers) + maybe the fact that those SSD are probably super smart and prefetch the next sectors? (see the jitter numbers I get even on a slow device in other comments)