I have data that works well in j files and key files but for possible performance reasons memory mapped files make more sense. At one time J did have MMF's for boxed arrays but I think it was discontinued. I think JD has this feature now.
What are the best options for memory mapping boxed arrays in J?
Arrays with a min to max column count ratio of 1/100 seem best suited for boxed arrays. I recall reading in the JD docs that the recommendation to consider switching to boxed arrays, using JD, was 1/50.