Before the new version, I played a 15% research game and didn't encounter the raiders for 104 years. I was able to fend them off, without losing any worlds, but mostly because my standard patrol craft using particle guns was just barely faster than their own craft.
I would suggest designing craft to accompany cargo fleets, something with a similar range and a long deployment time, and note that the raiders are usually lightly armored and will often avoid engaging larger military ships. I use 3-4 missile destroyers in a cargo fleet with 2mil km range missiles and a good agility component, just make sure to have sensors and fire controls with a resolution set for very small targets for defense. For hunter-killers of pirates, I typically use energy weapons and fast ships to intercept with instead to save on missiles instead.