It's 60% faster (0.6x faster, 1.6x as fast). Throughput (“how much stuff you can do per unit of time”) is the only reasonable way to describe performance[1], otherwise large speedups are basically impossible to describe in an intuitive way. (If something that took 10s now takes 1s, that's “ten times as fast”, not “zero point nine times faster”.)

[1] Even latency changes are best described in terms of throughput.