Not sure what the difference is between V6 and V7, but between vanilla and a mod pack (DW2 / Mindcrack) is huge. If you whould have used the search function you whould see your not the only one asking questions about it.
Simple fact: The more mods that use some kind of tick count / tick update / graphics it lowers the FPS of the client.
That said the DW20 and Mindcrack have alot of mods in general and several that use the tickcount, this decreases the overal performance (FPS for example) of the client (Player).
So dropping from 70+ to 30 - 30 ish is alot but not really, looking your using a 670M GPU, i reckon your on a notebook, otherwise it whouldn't make sense to have such a huge FPS drop. My I7-2600k with 8GB and a 5970HD card drops from 120+ FPS (vanilla) to 60 / 70 fps (DW20V4). This both in single player and connected to a server on LocalLan