In couple words
Server performance due to overpopulated trees/forests/stumps
The issue usually is not on user's end, but on the server that you are on. Some servers start out really good and as time progresses they will slowly become more choppy and laggy.
There are a few things admin can do to help with that. The biggest one is (http://lifeisfeudal.com/forum/delete-poor-quality-trees-monthly-auto-uproot-t8393/)
This helps big time but drop the quality removed from 80 to 10. That will help big time!
If you are not admin or can't reach one, then try to make sure you remove ALL stumps in your area and thin the trees out as well. When you plant trees keep them 2 blocks apart and that also helps.