There was no way my opponent thought I was slowplaying them. It was clear that it was a complicated turn at the end of the game, and that I was thinking about how to play it. Slowplaying looks completely differently. Slowplaying is making each click super slow. I was never doing that.
It actually happened to me again right now. The player asked something (in Japanese), I said that I was sorry but the turn was very complicated, then he resigned me. The horrible thing is that the interface gives you no warning. At least tell me when I only have some seconds left!
And to answer josh bornstein about what kind of kingdom can make this happen. This one involved Crown, King's Court, Fairgrounds, Pilgrimage and Stonemason. On my last turn I had to figure out what to KC and Crown, what Crowns to save for the buy phase, whether to trash something with Stonemason, whether to buy Stonemasons, what to use my Pilgrimage for (token was facedown), what my Fairgrounds were worth if I trashed, kept or gained certain cards - and all of this because I might have the opportunity to end the game before my opponent's next big turn. I was one VP down at this point. Reloading the game, I see that I actually could have won by Stonemasoning a Gold into a Duchy and a Crown, Pligrimaging a Crown and a Haunted Woods, Crowning a Silver, and buying two Stonemasons to empty the Crowns and Haunted Woods for a three-pile. This is like a puzzle that can't always be solved in 4-5 minutes.