After making this post, you can well imagine how pleased I was to see the treants that spawned tonight, were NOT in towns, but out in the middle of nowhere!
Not only did it provide more fun and excitement from having to find where they were, but then to have to race to the spot to try and collect parts.
I'm very happy to report, that the lag was the same as any spawn of mobs in the wilds. fps stayed 16+ and I was never lagged out.
Together with 2 other rangers and a healer, we killed 10-12 lvl 120 blighted treants and was alble to get all the parts for a buddy.
Thanks for spawning them away from towns!