How many people have you tested it with? How much exploring has been done? It gets exponentially more demanding as those two numbers go up. That's why I stopped running mine. Got tired of dealing with the hassle.
So far, I've been able to support up to 8 players online with no lag. A
SIGNIFICANT amount has been explored (server used to be run on my home computer for the past 2 years). It is currently running on one of the super computers I have at work. I don't believe it is lag prone. Old server chunks were imported to a new seed, so strongholds, new biomes, etc. will all spawn without losing any of our old explorations or structures.
In preparation for the new patch that just came out (and the new influx of players since I'm advertising it here), I've moved the spawn to a new location, so you'll be exploring new chunks. I'll eventually build a rail to the old world though
