Do you have a good reference for sims merge sort? The only examples I found are pairwise-merging large numbers of streams but it seems pretty hard to optimise the late steps where you only have a few streams. I guess you can do some binary-search-in-binary-search to change a merge of 2 similarly sized arrays into two merges of similarly sized arrays into sequential outputs and so on.

More precisely, I think producing a good fast merge of ca 5 lists was a problem I didn’t have good answers for but maybe I was too fixated on a streaming solution and didn’t apply enough tricks.