Like even in magic formats that allow proxies, things like Black Lotus are banned, because they're too powerful.
ISTR reading "Shahrazad" was banned because it made game too long to play, rather than because it was too powerful. Any Dominion equivalent? Kings Court+Possession, perhaps?
Shahrazard is a bit of a special case. It forces you to play a subgame with your remaining cards, for a small (well, large relative to its cost) benefit. The issue is, that subgame is likely to take as long as a current game is to finish, AND it requires extra space, AND you can start nesting them. I forget what the limit of duplicates is in Magic, think it's 4, but even without any cards to get things back from your graveyard, you could do this:
Main game
-Shah 1
--Shah 1.2
---Shah 1.2.3
----Shah 1.2.3.4
-----End Shah 1.2.3.4
----End Shah 1.2.3
---Shah 1.2.4
----Shah 1.2.4.3
-----End Shah 1.2.4.3
----End Shah 1.2.4
---End Shah 1.2
--Shah 1.3
---Shah 1.3.2
----Shah 1.3.2.4
And so on. Yeah, each subgame gets a little quicker, but in terms of space needed explosion and potential to massively increase game length, it's just not acceptable.
I don't think there's anything that really comes close in Dominion. Almost always, Dominion works towards a fixed end - piles running out - and very few things progress your position without advancing towards that end. The only real possibility is a perfectly reliable Monument based Golden Deck stalemate, where e.g. the game is close to piling, both of you earn the same points, the score is close enough that each player takes the lead on their own turn, and nobody can add any more useful cards for fear of ending the game. And... that's a LOT of conditions on what has to happen, before even looking for kingdoms where it could be possible. I guess there might be a few more edge cases where it happens, but you know eh.
The only other case where the time it takes could be too large is if the time spent playing cards compared to the rate of working towards the end of game is too low. By which I mean e.g. a lot of cards are played, but only 1-2 cards are taken from the supply each turn. It's hard to imagine something like this really happening, though.