MSMQ sends small packets, tinygrams, which experience a 300ms delay before being sent.
The Windows NT TCP/IP stack uses the Nagle algorithm, defined in RFC 896, to prevent small packets from flooding the network. The algorithm only allows one un-acknowledged small packet per connection. This conflicts with MSMQ's tinygrams.
When SP6 is released, you can disable the Nagle algorithm for MSMQ by navigating to:
On the Edit menu, Add Value name TcpNoDelay, as a type REG_SZ. Set the string value to 1. Stop and restart the Microsoft Message Queue Service in Control Panel / Services.
NOTE: No, I don't know when SP6 will be released.