I will explain how it will work in b15 since it's more relevant, since it's changing slightly.
Blight spreads, and any adjacent block gets damaged over time, so the town WILL crumble.
Purge crystals will not only remove the blight on their aoe, but prevent blight encroachment.
Yellow has a small radius, blue larger, red largest.
The idea is to chase you out of town... Let's face it, it's a lootfest and the entities can be cheesed like Vanilla ones can (nerd poles, roofs, etc). Blight can't be cheesed. =)
So, you get all comfortable in town, building your base, thinking you're hot ♥♥♥♥, then the blight comes and you end up running like a little girl with only the things you can fit in your backpack and saddlebag. Hopefully you have enough stuff, but in likelihood you won't. =)
Now you're out on the Frontier, blight chases you on a more regular basis because the game stage brings the specific entities, you know now that it's foolish to establish a base, and now you have a purpose. Defeat the blight.
Building yourself up strong enough along the way, so that you can travel to the crystal biome and get what you need.
Will it lag? No.
...hope this helps.
In theory you can recognize and target the specific entities that cause the blight, and actually prevent it from spreading far before you can contain it. But that is going to take playing the game differently than you're used too, as promised, and making different decisions.