It won't be instant, because the keys are dispersed in range 1 to 2**256, so, as I already mentioned, this requires a dictionary data structure (map, tree, hash table, database are such examples).

The only way to have a O(1) lookup is, by basic principles, to know exactly the location that needs to be read. Since you have 2**30 keys, and each key is 256 bits, a truly O(1) lookup requires 2**256 * (sizeOfAssociatedValue) bytes, in order to do a direct O(1) lookup of any 256-bit key. You will have 2**30 locations that have an associated value, and (2**256 - 2**30) locations that are wasting space for nothing. But it is O(1) in complexity.

That's why other data structures like trees / maps / hash tables / storage databases are optimal: they are the perfect balance between minimizing storage and minimizing complexity down from O(n) to O(logN). O(1) is only practical if there is enough spare memory to keep all possible keys.

There are only around 2**70 bytes in all storage devices ever manufactured on Earth, so do you get the picture now?