Hi,
I was facing same problem about 2 -3 back until the day I fed up and restored as new iPhone and restore my recent back-up; which doesn't help much. Same lagging, freezes while switches screens, opening apps, slide to unlock, search iPhone screen, settings, camera app... and it goes on :(
So finally I restored my phone as new iPhone with no back up to be restored, like a teen. I must say somehow response is faster than previously. I used for like 3 days with no apps and music into it, now same slowness is back when I installed just few apps (iPhoto, twitter, Facebook, find my iPhone, whatsapp, viber, im+ pro and iBook).
Also I called upon apple care customer care, they suggesting to get a replacement. Now I don't understand my iPhone was smooth and blazing fast when I bought it and tell me tell you I didn't even happen that I dropped my iPhone & something like that which could affected hardware of iPhone.
Read almost all the articles over this but all in-vein.