I think the old duplicator still works? That's the one that just has the glowing effect and stuff.
The thing is, how many bricks are you trying to duplicate? I remember trying to duplicate something like 5k bricks in my lounge (we had a huge pool that we needed to mode). And we had to sit for a few minutes before the server could actually do it. When we placed it, it actually crashed all the clients and we had to rejoin.
If the builds you're duping are this big, the highlighting probably isnt the problem but the searching through the bricks it (the duplicator does a sort of recursive search, checking all bricks above and below every brick it encounters so that it can travel up and down zigzags and find EVER brick). Once it gets all the bricks, it's not too intensive to highlight all of them. The duplicator has a mechanism to not ghost large amounts of bricks.
If you need a special searching algorithm (one that doesn't do as much searching, but definitely isn't as flexible), I could delete some code or show you what to do?