How can I troubleshoot slow Internet speed in SonicWall firewall in two steps?
03/26/2020 
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DESCRIPTION:
A common problem we come across "Internet speed is so slow... crawling.... We have a 100 meg up and 100 meg down speed from ISP.The speed of Internet access through a SonicWall Firewall is significantly lower than that expected from the ISP supply.
The purpose of this article is to discuss common configuration issues. Links to articles for further reading are provided towards the end.
CAUSE:
While slow Internet access speed through a firewall can be caused by a number of issues, it is worth checking a few configuration tweaks that have helped resolve the issue in several occasions.
RESOLUTION:
- Check lInk speed WAN interface (e.g., X1) | Advanced:
TIP: Fig. 1. Optimizing the Link Speed and MTU on the Advanced tab of the WAN interface where the defaults fail to establish a compatible ISP connection.
- Check the MTU.
A mismatch in the maximum transmission unit (MTU) between the firewall and the ISP device can impact the bandwidth. In one of my cases, just by optimizing the MTU we were able to regain the bandwidth (5% to 90%). An optimal condition for the test would be to connect a computer directly to the firewall (Fig.2). This is to rule out uncertainties about latency due to a network. 
TIP: Fig.2. Checking MTU on a directly connected computer is my preferred way to minimize uncertainties about latency involved in a complex network.
A typical MTU optimization test involves doing a ping with the options of -f (don't fragment) and -l (size) as summarized in Fig. 3. I start with an MTU of 1500 and find out a value where there is a successful ping. Then I add 28 bits to derive an MTU value I would be using on the WAN interface.
TIP: Ping Test on a Windows Computer directly connected to the Firewall. I would add 28 to the final MTU value that resulted in a successful ping.
- Check the DNS settings of the Computer where you are testing.
On a few occasions, although you see a reasonable bandwidth, web pages take long to initially load. First suspect was the DNS. When you type www.abc.com on a browser, the first thing we do is a DNS name resolution to get the IP. In Fig. 4, the host is trying to resolve names by accessing local DNS- one local and another across a VPN tunnel. This is a security measure. However, if the local DNS is having an issue (overwhelmed, network latency...), it will slow the DNS name resolution. Just by using a public DNS that is readily available, we were able to overcome the slow page load issue.
TIP: Fig. 4. DNS set on the Network Configuration can often create slowness in initial lading of web pages on a browser. To rule out latency due to name resolution consider using a public DNS that has a fast access.
Further Reading:
Following is a compilation of related articles for further reading. They include involved troubleshooting methods and scenarios.
Troubleshooting Network Throughput, Latency, and Bandwidth Issues with a SonicWall UTM
Optimize MTU for VPN
Minimum Bandwidth, Latency and Keep Alive for a Tunnel Client Connection
To troubleshoot speed or throughput issues with the SonicWall
How to use iPerf to measure Throughput on a SonicWall device
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