Redis caching in nodejs

So I was looking at this module, and I cannot understand why it uses callbacks.

I thought that memory caching is supposed to be fast and that is also the purpose someone would use caching, because it's fast... like instant.

Adding callbacks implies that you need to wait for something. But how much you need to wait actually? If the result gets back to you very fast, aren't you slowing things down by wrapping everything in callbacks + promises on top (because as a user of this module you are forced to promisify those callbacks) ?

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