constraints using and performance question

I've heard an opinion that adding foreign keys or check constraints at
runtime can be a huge performance bottleneck. (I mean for example
foreign keys in fact table and runtime process of data loading to data
warehouse database)

I don't think so, in my opinion using at least foreign keys is always
good practice and data consistency is ever more important than time of
data loading which is relatively rare.

Has anybody an experience of huge performance bottleneck during
process of data populating?

Thanks an advance,
Andriy

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