Universal Media Server | Forum

It sounds to me like they're transcoding. If they take "forever", you computer might be slow? That said, if that's the case, they should lag also after starting playback.

A "gen4 i5" ranges from very slow to quite fast. You need to specify how long "forever" is, and you need to post debug files as described in red above for anyone to look into what's actually happening instead of just guessing.