Yeah, don't go deleting any folders or files, and even then, moving folders can cause... headaches... Best to just download the pack and work around the limitations.
A couple of questions here before I give suggestions.
1: What version of modded MC are you trying to play? 1.6? 1.7?
2: How much ram are you actually allocating to the pack?
3: What version of java are you using?
When it comes to question 1, this is important, as the 1.7 packs are fairly unstable still, and all could (should) use of the Fastcraft mod (found here:
http://forum.industrial-craft.net/index.php?page=Thread&threadID=10820 ) The mod is not available for pack distribution though, so you would have to download it and add it yourself.
Question 2 is a tricky one. Unlike other "games" modded MC actually gets worse the more ram you give it. Try to keep your ram around the 2-3 gig mark and see how that works. I have 24gigs of ram, but only allocate 4 to MC.
Question 3 is also related to question 1 here. Depending on the version of mod pack you want to play, you may need a different version of java. Java7 is good for the 1.6 packs, but some have found it inadequate for 1.7 and have updated to the latest version of java8. Also, any mod pack after, and including 1.6 required 64bit java to run. If you are using 32bit java, you're gonna have a bad day. You need to get 64bit java.
Some have found java parameters help as well, but those can be a bit of a crap shoot. What may work for some, may not work for others.
The only other suggestion I've seen for making modded MC work better (1.7 version) is to use a program like MultiMC to "re-create" the mod pack you desire (found here
http://multimc.org/ )
I haven't tried it myself for 1.7 packs, but I have read people are experiencing smoother game play using this to launch the packs.
Good luck!