From what I gather the current lag issues have come forth in the most recent update to several of the modpacks. I to have experienced said lag in all of them up until today. Today, I've found a solution that has improved performance significantly, but again is not guranteed to work. This is a temporary solution until the dimension related mods sort their issues out. The theory that ChickenChunks causes the problems is a complete failure in my experiences. Some of you may have had different experiences, I am not here to argue, but provide a solution that may or may not work. Within the options menu, click advanced options. Type in -XX:MaxPermSize=256M within the additional Java Parameters spot. My own theory on whats causing the issue is Java itself. How you may ask? Simple, I am a developer and use Eclipse on a daily basis. Eclipse too has begun experiencing crashes. My suggestion is to wait for a Java update.