[12:47:16] [Server thread/WARN] [FML]: Ruins Mod loaded a new chunk [-25, 21] in dimension 0 (overworld) while populating chunk [-24, 22], causing cascading worldgen lag.
[12:47:16] [Server thread/WARN] [FML]: Please report this to the mod's issue tracker.

Yes, it's the nature of Ruins. As it spawns in structures, these may exit the "already generated" chunks, which makes Minecraft generate them, which in turn causes more worldgen, which may trigger more Ruins. It's very unlikely to cascade too far, and in the end it does not cause more overall lag - it just causes worldgen to frontload more.