The most likely cause of slow syslog logging is that the SonicWall does not have a DNS entry for its IP address. Some syslog servers try to use DNS to resolve the IP address of the device sending log information. This can result in delays of 30-60 seconds until the DNS lookup times out and the entry is logged on the server.
There are two possible solutions to this issue:
Assign a DNS name to the IP address of the SonicWall. If the SonicWall is in NAT mode, assign a name to the IP address (WAN or LAN) by which the syslog server will see the SonicWall. This depends on the location of the Syslog server. If the Syslog server is on the WAN, assign a DNS name for the SonicWall's WAN IP address. If the server is behind the LAN, then the LAN IP address should be used for DNS assignment.
Configure the Syslog server software so it does not attempt DNS resolution.