why speed test results are usually lower than advertised speed

Overhead

• Your internet speed is measured in raw bits per second (bps), but real-world data transfer includes additional information like headers, acknowledgments, and error checking.

• TCP/IP, Ethernet, and other protocols add packet overhead, meaning some portion of the bandwidth is used for network management rather than your actual data.
• This overhead typically accounts for 5-15% of the total bandwidth, which explains why you rarely see a full 1 Gbps in speed tests.

Speed test client and server

The speed test server’s capacity can impact results. Some servers may be congested or unable to fully utilize your bandwidth.

The distance between you and the test server affects latency, which can slightly reduce speeds.

The speed test client needs to have a good NIC (intel if possible) and good spec (RAM and CPU) because it needs to make many connections to test the connection. For example if you test with a 100mbps NIC that is the max speed you can see. You need a 1gig NIC or better to test a 1gig connection.

ISP and firewall/network management and contention

• ISPs often use network shaping, congestion control, and peering agreements that affect speed.
• During peak times, ISPs may limit speeds slightly to ensure fair distribution of bandwidth among users.
• Your internal network/firewall may do the same
• Its best to test out of hours with just your test laptop plugged into the internet connection to give the best results

check a DNS TXT record

 

https://mxtoolbox.com/SuperTool.aspx?action=txt%3a%40.dlrcoco.ie&run=toolpage#

nslookup -type=TXT mail._domainkey.domain.com

dig TXT domain.com +short

Powershell

Resolve-DnsName -Type TXT domain.com

Meraki monitoring pages

Organisation -> Alerts

Network-wide > clients

Network wide > Traffic