In my tests serializing/unserializing a 1.5MB array was as fast as including a file with that array, which was much faster than JSON..for some surprising reason.
I benchmarked it a bit back and found that it was fastest to json_decode the array but the entire array was nested arrays of numbers, and we're talking a 26k string when JSON encoded. (Remember, for the majority of the time, the hurt of serialize isn't an issue, it's the hurt of json_decode vs unserialize vs raw array that we're interested in)





