It's going to be based on the number of cards in the set. There's probably a particular threshold (a few hundred cards), over which the act of creating the cardlist takes more than the 30 seconds that Heroku allows before they force a timeout error.

Fortunately I do still get to continue rendering, in the server process, I just don't get to send the result to you; but I do get to store it in my cache, so that when you refresh a couple of minutes later I can serve it to you quickly.

But yes, taking the cardlist of big sets (say >300 cards) isn't going to work well. In general searching the cardset will be a lot faster if there's something particular you're looking for, though I appreciate that's not as user-friendly. The visual spoiler I think does work reasonably well even for huge sets, too, because it splits them into manageable chunks.

What I should probably do is make the cardlist page load quickly, and then send a series of Ajax requests for 100 cards at a time. Well... what I should actually do is fix the performance so that the cardlist doesn't take tens of seconds to render, because being that slow is pretty inexcusable. But I haven't thought of a way to do that yet.