Are They Really Slow?
Maybe they seem slow because your group is playing fast. Before you blow a gasket, be sure they are really dawdling. You can find this out easily by checking your own elapsed time at the turn. If you've completed nine holes in two hours or less, you don't have a lot to beef about. If the group in front of you is moving along at a reasonable clip, maybe you should consider slowing down a bit. They don't deserve to have you pressing them if they're playing a four-hour round.
Are They Really the Problem?
When you encounter a slow group, before you decide to yell at them, make rude gestures, or send a missile booming over their heads, check carefully to be sure they really are the problem. What if someone is holding them up?
The Danger of Confrontation
You never know how strangers will react- on or off the course- to your pointing out their poor behavior. Before you confront people, always ask yourself whether it's really worth it. If you do ask them to do something- such as speed up their play, let you play though, or stop pressing you- and they decline, be prepared to back off. It's simply not worth the possible escalation of the situation.