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

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.