What do you mean 30ms max? You want to send a string every 30ms? Or you want a broadcast to take only 30ms to get from one end to the other?
If it's the latter, then there's not much you can do. ZigBee is a slow protocol because it has a lot of overhead due to it being a Mesh protocol.
If you want a faster protocol, take a look at XBee 802.15.4.
However, even the fastest transmission are usually around 12-20 ms, and any failures and re transmissions will cause more latency. RF is not designed to match the low latency of a physical connection.