1.7x faster means oldRunningTime / newRunningTime = 1.7. Or in your example, 8 seconds / 2 seconds = 4x faster. Sometimes this is called a speedup of 4x or a 4x speedup.
I totally get how the math works. I don't like the language. We're not measuring speed or velocity which can be represented as rates. We're measuring time. Speed would be something like operations per second. We're just measuring seconds.