Why is it that you think the tectonic changes were always so slow?
1. They're slow now
2. You presume that sediments alwyas collect at today's rates
3. You assume that radioisotopic decay has been constant
While I'll grant number 3 as being on a stong footing, number 2 a priori assumes no flood so it can't be used as evidence and number 1 similarly assumes uniformitarianism.
So we're really down to radiodating. Apart from radiodecay there are no real reasons that these processes couldn't have happened much more quickly than you assume. And on the radiodecay front we have evidence of excess helium retention suggesting accelerated decay.
For us of course we even have the Bible telling us in plain language that the doubters will 'willing forget' that the flood occurred.
I completely understand your point of view. If there was no flood, no accelerated decay then of course it happened over millions of years. We're simply sharing an alternative way of looking at it that is unashamedly Bible inspired.