I have encountered a similar problem when fighting Delin.
He casts fear at start of the fight, I can see the animation and hear some sound then a 20s wait then another animation over my heroes showing what effect the cast had and sound for resists.
I think it's a issue with spells or magic attacks, calculating their outcome. One of my heroes uses a magic ranged staf. When she casts there's always a 20s gap between her cast animation + sound and the animation + sound that show if the attack hit or miss.
I play on an Phenom 2 955 , 4GB RAM, Sapphire VaporX ATI Radeon HD 5770 graphic card.
I can provide savegames to see both issues in action.