I think when people measure speedups here they deduct the first 100%, e.g. if I used to be able to process 10 items per second and can now process 20 items per second you can run 200% as fast but it's a 100% speedup.

To avoid the various ambiguities here, I learned to express speedups as a ratio of the optimized throughput, divided by the baseline throughput. Or equivalently: the baseline time divided by optimized time.

4400 MB/s vs 800 MB/s = 5.5x speedup or 5.5 times as fast.

A three digit percentage means at least a quarter of the time, but possibly more.

Something that is 50% faster will take half the time. Something that is 100% faster will take a quarter, and so on.