If your question is purely "is the value prime" the Fermat primaility test is very fast, probably about as fast as a hash table lookup on a table that large. Ru

If your question is purely "is the value prime" the Fermat primaility test is very fast, probably about as fast as a hash table lookup on a table that large. Running Baillie–PSW to eliminate false positives isn't a huge deal either as it will only need to run if Fermat passes. It's not a horrible idea on its face and may indeed be a little faster, at the cost of lots of memory. I don't know of a use case where it would be worth it.