Slow Office Network? Eight Checks in the Right Order

“The internet is slow” is a symptom, not a diagnosis. Behind the same sentence you can find a damaged cable, a crowded Wi-Fi channel, or a server quietly running a backup at the wrong hour. In our support queue the first call almost always goes to the internet provider, while the real cause is usually inside our own walls.

The order below starts with the cheapest, fastest check and moves toward the expensive decisions. Measure first, then touch things.

Is it everyone, or one person?

This question can save half a day. If one person complains, the problem is likely their device, cable or Wi-Fi signal. If everyone is slow at once, look at what they share: the internet line, the firewall, the core switch, or DNS.

Time matters too. “Slow at nine, fine in the afternoon” points to a load pattern: everybody signing in and pulling updates at the same moment.

Eight checks, in order

Step	Check	How	Typical finding
1	Line speed	Speed test from a PC cabled straight to the modem	Line far below contracted speed
2	Cable and port	Check link speed; 100 Mbps instead of 1 Gbps means a bad cable or port	Crushed or over-long cable, old port
3	Wi-Fi channel and signal	A scanner app showing nearby networks	Many networks on one channel, weak signal
4	Core switch load	Port statistics in the switch management page	Port with rising errors, saturated uplink
5	DNS response time	Query the same name through two different DNS servers	Pages “wait” while the speed test looks fine
6	Background traffic	Top talkers on the firewall	Backups, updates, cloud sync
7	The device itself	Disk, memory, startup programs	Full disk, aging computer
8	Firewall capacity	CPU load, session count	Appliance cannot carry the line speed

Do not skip ahead. The most common mistake we see is replacing the expensive firewall at step 8 before trying the cheap cable at step 2.

Cable and port: boring, and the most frequent cause

If a computer shows 100 Mbps where it should show 1 Gbps, some of the eight wires in the cable are not healthy. Gigabit uses all eight, while 100 Mbps uses only four. A cable pinched in a door, crushed under a desk leg, or terminated by hand produces exactly this symptom.

Length is a limit as well: copper Ethernet is rated for about 100 metres. Beyond that you need an intermediate switch or fibre.

Wi-Fi: five networks in one room

In multi-storey office buildings the 2.4 GHz band is nearly always crowded. Only three channels (1, 6 and 11) do not overlap there, so a neighbour’s access point can drown yours. The usual fix is to keep office devices on 5 GHz, plan the number and placement of access points once and properly, and choose channels deliberately.

Swapping one access point for a “more powerful” one rarely helps. The problem is placement and interference, not power.

DNS: speed test fine, pages slow

We overlook this symptom most often. The speed test looks healthy, yet the browser waits two or three seconds on every new site. The cause is frequently a slow DNS server on the network. If the DNS role on the domain controller is unhealthy, or devices are left on the modem’s own DNS, everyone is affected. The test is simple: resolve the same name through the current DNS and then through a well-known public one, and compare the times.

Invisible traffic: who is eating the bandwidth?

Open the firewall’s “top talkers” view. This list shows you the backup that starts in the daytime instead of at night, the single PC distributing an operating system update to dozens of copies, or a cloud sync re-uploading a 40 GB folder. The fix is often not more bandwidth but scheduling the job for a different hour.

Frequently asked questions

Will a faster line fix it?

Only if the speed test shows the line is actually full. If the line looks half empty and the office is still slow, a faster package changes nothing except the invoice.

When should I get outside help?

When steps one and two are done, the problem still sits at a shared point (switch, firewall, DNS), and nothing is recording history. Without monitoring, every diagnosis is a guess. Where remote monitoring exists, the same problem is typically caught before users notice.

The slowdown is temporary. Why bother?

Temporary slowdowns are often the first sign of a failing component. A port with a rising error counter becomes a port that stops linking a few weeks later.

Your first three moves

Write down who is slow and at what hour. Then cable a computer straight to the modem and measure the line. Third, log in once to the core switch and the firewall management pages and see which port or device stands out. In most offices the answer appears within these three steps.

If you cannot find why your network slows down, or you want a recurring problem monitored permanently, get in touch with us.