That's a common issue. Phyre is right, it mostly occurs during birthday parties. You may have noticed that a Sim will take ages to finally age up during a party with everybody cheering, but a single Sim with no Sims around will age up nearly instantly. Other interactions can be treated the same way. You just have to wait for the blocking Sim/interaction to stop, otherwise you can restart the interaction. Of course, that can't be a solution with birthday cakes.
That problem is in everyone's games, I can assure you. Most people have been complaining about this.
In addition, such delay also happens when the fire alarm at the Fire Station rings (Firefighter profession - Ambitions) and your Sim just stands there doing nothing for forty minutes, while the performance meter goes down. That's simply called goofing around.
I believe it's just an annoying glitch/feature in the game and has nothing to do with hardware or computer.
Good luck in finding the solution.