Havn't tried lowering resolution. Probably should though, but considering good initial performance I am led to think this is an optimization issue rather than a hardware issue.
Distance of Drawn Objects and ammount of objects appear to have direct impact on speed of effect/degradation, but initial performance usually the same.
Not sure true cause, whether my side, or server side. Not sure what effect zombies have on this, probably none, as effect can occur in middle of forest with no chasing zombies. Occurs equally on empty servers.
Game otherwise totally stable. No CTD or anything.