This strongly depends on the transformations you are doing while loading the data. If you are simply renaming a couple of fields, then an unoptimized load might be insignificantly slower (depending on data size).
Generally optimised loads are a good indication of a performant loading processes, hence why should always try to reach them but same time not at all costs.
e.g. it might be very useful to reduce the RAM footprint by loading unique fact table keys using AutoNumber() at the cost of breaking an optimised load.
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