You had it right the first time, 1.7x speed is 70% faster.

If something previously took 4s now takes 2s then it's 100% faster.

Think of driving 10miles. If you drive at 20mph then it takes 30 minutes. If you drive twice as fast, 40mph, it takes 15 minutes.

40mph is 100% faster than 20mph.

Half the time is twice as fast!