Why is my Internet rubbish ?
There are many things that can affect the quality of your connection to the rest of the world.
The connection to your property
Over the years the connections available have improved from Dial-Up, through ADSL, FTTC, 3G and 4G to a Fibre connection.
Some lucky people have a choice for their physical connection to the Internet, and to see who you could use enter your postcode here https://www.thinkbroadband.com/broadband/availability.
For most this will be provided by OpenReach, who run what was BT's network, and the latest information on what they can offer you is here https://www.openreach.com/#checkavailability with the expected maximum speed.
You can check your actual speed here https://www.speedtest.net or here https://fast.com and if what is reports is well below what OpenReach say it could be, contact your provider and ask "Why?" Can they upgrade you to the faster service? You can move to someone who will provide a better service.
If you have the best service available, and you aren't getting the best speeds or the line is laggy / you experience buffering or break up on VoIP phone calls, this could be due to delays on your providers' network. Think Broadband run a monitoring service and you can activate it for your line here https://www.thinkbroadband.com/broadband/monitoring/quality, but you do need a static IP address / one that doesn't change while the monitoring is in operation.
Once you have got the best speed you can and your connection is reliable you need to look closer to home
The connection within your building
Cable is the most reliable, if you can turn off Wi-Fi when connected via a cable to make sure you are using the better of the two connections.
If you are using powerline or similar extenders, then they will help, can be unreliable, and they are not as good as running a cable back to your route / firewall.
If you are connecting via Wi-Fi how strong is the signal?
Are you connecting to the Wi-Fi 6 signal? Rather than the 5Ghz one but that is still way faster than the basic 2.4Ghz signal
If there are lots of other Wi-Fi networks covering your building, are you on the right one, is it frequency optimised to avoid the others?
Which of course leads onto
Who else and how are they connecting to your network?
If there are people playing on X-Boxes / PlayStations streaming films in HD that will use up lots of your bandwidth and leave you little for your work needs.
If you are connecting via Wi-Fi who else is sharing the capacity of that point? The typical 300Mbs it claims normal only connects at 150Mbs or 72Mbs as you move away from the transmitter, and if that is split among 10 or 15 other devices it is easily only 5Mbs each, when your building is provided with an 80Mbs connection, you can see the poor performance is under your control.
How to Fix it?
Wire back to the router / firewall data cables, they can be long patch leads just lying along the corners of your rooms going to additional Wi-Fi point, or to ceiling mounted Wi-Fi access points providing full building coverage.
Fing shows what is connected to your network
WiFi Analyzer shows the Wi-Fi channels around you. Helps you to find a less crowded channel for your wireless router.