Are all your flights on the same airline? If so it will be easier to go from terminal to terminal. We had a different airline for germany to LAX so we had to leave the secure area, then go back through the security check point it was a pain in the bass. Had I booked the same airline I could have just gone down three gates. But hey I saved money.
You don't have to worry about your bags, they will move to the next plane without your help (hopefully). If you are on a different airline you want to be sure which is your next gate is and hightail it over there quickly. Some of these security check points are a pain.
You do clear customs in JFK, I can't remeber if we grabbed our luggage or not
. Customs in LAX was quite a bit faster than I expected. BTW I avoided JFK. It's a large messy airport hard to get around. Have you purchased your tickets yet? If not look for alternative routes. We did a Transatlantic cruise so I only had to put up with that headache once.