I think that the question isn't slow or fast, it's exponential or not exponential. If you are getting 9 infections for every infection then that's a bit exponent. But, 2 for every infection is also an exponent - just a smaller one. Once you get below 2 for every infection then you are in the territory of slow growth - but not for long and not everywhere. The UK had a patchy delta wave, my guess is that when the non herd immune demographics and areas started to get herd immunity (drop below 2) then the breaks really slammed on.