[foota]

I think they meant it only runs quickly enough when everything's in cache? But that doesn't make much sense to me. It's really hard for me to believe it'd be I/O bound though, when memory bandwidth is like tens+ of gigabytes per second?

[imtringued]

IO bound can also mean that memory latency is a problem. It's not difficult to believe that DF is doing lots of pointer chasing.