1.8 is going to be a long time from now. 1.7 could turn out to be a long, painful experience...
I've done some testing in single player. These are the mods which I've focused on, specifically for their world generation components:
Battle Towers, Biomes O' Plenty, Farlanders, Hardcore Ender Expansion, Natura, Roguelike Dungeons, Ruins, and Twilight Forest.
Removing all of them leaves me with almost no non-vanilla worldgen. Exceptions include ores from Thermal Foundation, abyssal stone from Railcraft, meteors from AE, some trees from HarvestCraft, and bees from Forestry, as removing them pretty much guts the pack. In this slimmed down pack, chunk lag is noticeably better. It is still there, but pauses are no more than half a second, compared to 10-20 full seconds before. Adding Natura by itself brings back 5-10 second pauses, and adding Biomes O' Plenty by itself brings pauses of 3-5 seconds. Still testing the others.
I should note that this problem is exponentially worse in single player, so if you can tune your pack's performance acceptably in single player, then it'll be much better on a dedicated server.