Never noticed any speed difference due to bucket size. S3 is generally slow anyway (250ms for a write isn't uncommon) but it scales very well and we use it for raw data storage that's not in our critical path, so the latency isn't a problem.
Edit Response: I've always used the partitioning conventions they suggest so not sure what sort of impact you encounter without.
Edit Response: I've always used the partitioning conventions they suggest so not sure what sort of impact you encounter without.