-_NB_: `.SD[1L]` is currently optimized by [_`GForce`_](https://Rdatatable.gitlab.io/data.table/library/data.table/html/datatable-optimize.html) ([see also](https://stackoverflow.com/questions/22137591/about-gforce-in-data-table-1-9-2)), `data.table` internals which massively speed up the most common grouped operations like `sum` or `mean` -- see `?GForce` for more details and keep an eye on/voice support for feature improvement requests for updates on this front: [1](https://github.com/Rdatatable/data.table/issues/735), [2](https://github.com/Rdatatable/data.table/issues/2778), [3](https://github.com/Rdatatable/data.table/issues/523), [4](https://github.com/Rdatatable/data.table/issues/971), [5](https://github.com/Rdatatable/data.table/issues/1197), [6](https://github.com/Rdatatable/data.table/issues/1414).
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