> I tried an experiment where I memory mapped a large file and started updating bytes at random. I could get the rate down to kilobytes/sec.

Memory-mapped IO means you're only giving the SSD one request to work on at a time, because a thread can only page fault on one page at a time. An SSD can only reach its peak random IO throughput if you give it lots of requests to work on in parallel. Additionally, your test was probably doing small writes with all volatile caching disallowed, forcing the (presumably consumer rather than enterprise) SSD to perform read-modify-write cycles not just of the 4kB virtual memory pages the OS works with, but also the larger native flash memory page size (commonly 16kB). If you'd been testing only read performance, or permitted a normal degree of write caching, you would have seen far higher performance.