Packed Array Summary

The main point about packed arrays is that Mathematica uses them automatically where appropriate. This is very much the case for the linear algebra functions, which naturally work with the same internal representation. Thus many functions return packed arrays even if the input is not packed. For example, the function Dot returns a packed array because internally it works with the packed version.

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To make use of packed arrays, it is typically hard to exploit functions such as Developer`ToPackedArray. The main issue is to make sure that you have a uniformity of elements in your matrices; for example, that you are not mixing integers and reals.