I posted a question about this issue a couple of days ago on the Lift group to see if it was known--I did not receive any response so I am assuming it is not known:

In this example I am seeing a 12+ second render time
for a list of 50000 elements:

val decomposed = decompose(1 to 50000)
log.debug("decomposed")
val rendered = net.liftweb.json.render(decomposed)
log.debug("rendered")

resulting in:
14:16:42.401 [http-8080-2] DEBUG c.t.s.ServiceServlet - decomposed
14:16:55.147 [http-8080-2] DEBUG c.t.s.ServiceServlet - rendered
^^ 12+s elapsed

I have reproduced this in both M4 and M5 on a i7 quad and Snow Leopard--JDK is:
> java -version
java version "1.6.0_29"
Java(TM) SE Runtime Environment (build 1.6.0_29-b11-402-10M3527)
Java HotSpot(TM) 64-Bit Server VM (build 20.4-b02-402, mixed mode)