Probably the most stressful thing I think I can throw at my iMac is FCPX, and that never approaches topping out my installed 16 gb. Interestingly, I've the i5 processor and only two apps have ever triggered the fan - iMovie and Civilisation (whatever the latest one is). FCPX seems to use resources far better than iMovie, and that ended up being the decisive factor in me resisting upgrading my current 5K iMac.