Avoid cards that give out Curses (though it is good to let new players play with Curses, otherwise they will never understand that it's a part of the game). Sea Hag is particularly potent at drawing out game length because it can be bought early and it doesn't benefit the attacker. Attacks like Spy, Thief, Saboteur, etc. tend to draw out game length because it takes a lot of time to reveal cards, discard them, etc.
Avoid King's Court.
Kingdoms that don't take very long to play tend to be money-ish kingdoms where the best strategy is a BM variant or something using kingdom Treasures. Those tend to get boring after awhile. If you want something more engine-y that doesn't take an exorbitant amount of time to play, cards that don't involve a lot of thinking, like Stables, Lab, and Market, are good. An engine that requires terminal draw and villages takes more time to play than an engine that requires non-terminal draw.
Sifting cards, such as Cellar, Warehouse, Storeroom, Cartographer, etc. tend to take more time because of the decisions involved (and in the case of Cartographer, the physical action of looking at and discarding cards). Conversely, trashing cards tend to take less time because trashing cards usually means that you kill the rest of your turn, and you will never have to make decisions regarding the worse cards in your deck once they're gone anyway.