It's more complex than that... Sequential reads are faster than random reads across most mediums (including system ram), but if there's enough prefetching, you might get better throughput on an SSD with the data fragmented than contiguous because SSDs can often read from multiple regions in parallel and contiguous data might not allow for that.

If you're at the point where you're optimizing for this, you've got some really high performance requirements though. And you'll have to do your own testing, because rules of thumb won't do.