I'm thinking of groupby("time.year"), which looks like a resample, and also resampling type aggregations,

In general, if by is sorted, then blockwise is potentially the best option.

We could gate this on

1. 1D by
2. ???

I still prefer the idea of using preferred_chunks on xarray's new Grouper objects to rechunk it intentionally but a heuristic might be effective in the intermediate.

method="blockwise" automatically rechunks, so this would be step towards the above.