If you specify other keys splunk will use those keys to narrow down what needs to be looked at in the first place, that's what indexing data is for.
As a simple example, looking for sourcetype=x error will only consider events that contain the word error from that sourcetype, potentially cutting the amount of data that needs to be read from disk by a huge factor.
Personally, I see no point in specifying a query that loads and lists 50GB of data, but then I don't know what you're trying to achieve...
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