Hey guys!!! Been playing island paradisio for about two years now, basically my family trees are like 3+ generations and ive made about 200 new lots with houses so my game is soooo slow!
I cant part with it though wondering if starting a new game in a new world (and not deleting my original) would result in a slow, laggy game on the new one ???? Sorry if this is a dumn question !!!

It should not bother your new game. You can always copy your save file of your old game and put it somewhere else. Then when you want to play there again you just put your save game back in and play it. Just be careful about deleting any custom content I only keep one save in my game and that is the current game I am playing. The rest of my saves stay on my desktop in a folder until I want to play that town again.

Starting a fresh new game with the old world still in the game folder should not cause the new game to lag.
Try some of the suggestions in Tips For Better Game Performance on your old world and see if it helps performance in your current game.

Here's a suggestion you might want to consider:
I have a legacy game I've been playing for over 4 years. The legacy families, extended families and friends just got to be to many to manage even playing rotational. I set up a couple of new worlds, had some families travel to one of those worlds and then using Traveler had them switch homeworlds. I linked the worlds by dropping the .nhd folder in each of the original world game saves. The worlds are all linked with one another and families and friends can go back and forth to visit. If you are interested in trying that see Switching Home Worlds On This Page.