Wi-Fi is a great technology, but how you set up and use it can have a huge impact on its efficiency.
If you're constantly waiting for web pages in general to load, then before you complain that your ISP isn't supplying that promised 24Mb/s connection, spare a thought for what happens when that signal hits the airwaves in your house.

How it works

To improve Wi-Fi performance, we first need to understand what it actually is. Wi-Fi is also called wireless local area networking or WLAN in the management console of your base station or broadband router, and it uses a group of frequencies clustered around 2.4GHz to transmit and receive data between computers.
To ensure that data gets through, it uses a protocol called 802.11. If every network within range all used the same exact frequency of 2.4GHz, the various devices would swamp each other's signals - a bit like two radio stations transmitting on the same frequency.
To overcome this, the protocol allows devices to use 13 numbered channels, which all use slightly different frequencies to ensure that there's as little interference as possible.
Confusingly, there are a few versions of 802.11. The oldest is 802.11a, which is now obsolete. This has been superseded by 802.11b, which has a maximum data transmission rate of 11Mb/s. 802.11g, which is the dominant version in the UK, can transmit at a healthier 54Mb/s. The newer 802.11n can use two channels for a maximum of 300Mb/s.

Encryption

router
Security is very important in Wi-Fi networks, and this has to do with both logging onto the network and how individual packets of data are encrypted. The oldest Wi-Fi security standard is WEP (Wireless Equivalent Protocol). This was part of the original 802.11 protocol and has been cracked, making it insecure.
This insecurity came about because if enough packets can be captured from the airwaves, software can be used to work out what that password is. Several open source packages now exist (AirCrack, for example) that will attempt to solve WEP passwords, thereby allowing people to log onto your network.
An updated security standard called WPA (Wi-Fi Protected Access) was introduced in 2003, with a newer version called WPA2 coming along in 2004. This is still secure and uses a government-strength encryption algorithm to keep your networks safe. In some Wi-Fi equipment, the security used is referred to as RSNA (Robust Security Network Association). This is really just another name for WPA2.

CSMA

If every computer on your Wi-Fi network transmitted at the same time, they would jam each other's signals. To prevent this, every Wi-Fi network card sold (including the one in the base station) uses a technique called Carrier Sense Multiple Access (CSMA) to share the airwaves.
In CSMA, a situation called contention occurs when a Wi-Fi card wants to transmit a data packet, but hears that a packet is already being transmitted. It waits for a very short but randomly selected time before listening again. If the airwaves are clear at the end of that period, the card transmits its packet before listening again and subsequently transmitting the next packet if the airwaves are still clear.
This 'first come first served' scheme means that over time, all network cards get an equal opportunity to transmit all their packets. CSMA is also used in wired networks, and is a very efficient method of data transmission.
This being the case, it's usually external influences that are to blame for adversely affecting Wi-Fi networks. Before attempting to improve the performance of your Wi-Fi network, it's important to know what its performance is like before you start. Otherwise, how will you know for certain which measures work and which don't?

Set a baseline

speed checker
The easiest way of measuring current performance is to use an online broadband speed testing service. There are plenty available, and they all work in the same way. One service is Broadband Speed Checker.
First, ensure that your entire network is quiet. Turn off all streaming services such as Spotify (including killing the service in the system tray), all social media services, all torrent services, and all email clients that automatically update themselves. Check to make sure that the WLAN light on your base station is not flashing to ensure that everything is turned off.
From a Wi-Fi connected computer, click Start Speed Test and wait until the test completes. Rather than just performing the test once, collect several results over a few days. Try to run the test at different times of the day to see when the local loop from the nearest telephone exchange to the houses it serves is most congested.
Keep the results of these tests on a spreadsheet and you'll be able to see the best time of day to perform large downloads.
It's also a good idea to perform the same tests from a PC wired directly to the base station. This will give you a definitive measure of the difference in performance between wired and Wi-Fi connections.
Whenever you implement a change, re-test the Wi-Fi speed to see if there's any appreciable difference. You may be surprised to find that some simple changes can help you resist an upgrade to a supposedly faster connection.

Interference

The 802.11 protocol family uses some very clever low-level encoding techniques to ensure that regardless of circumstances, the signal stands a chance of still being heard over other noise, but anything we can do to help it will improve network performance. In some cases, such techniques can make a dramatic difference.
Everything would be fine if Wi-Fi network cards were the only things transmitting at 2.4GHz in our homes, but they're not. There are plenty of sources of interference that can cause the network cards to have to wait multiple times before being able to transmit their packets.
Incredibly, one of the biggest sources of Wi-Fi interference is your domestic cordless phone. If you have one in the same room as a Wi-Fi device, you can expect network performance to noticeably degrade every time you make or receive a call.
Bluetooth devices also use the golden frequency of 2.4GHz to transmit and receive data, and therefore also tend to cause interference on Wi-Fi networks. Microwave ovens are a domestic boon that we usually take for granted, but they're also a source of 2.4GHz interference.
Despite remaining perfectly safe to humans when cooking food, a microwave oven situated less than about 10 feet away from a Wi-Fi network card will degrade its performance. Don't forget that this 10-foot range can extend through walls into other rooms.
Added to these interference sources there are also less obvious ones. Your neighbour may be transmitting on the same channel, and have placed his base station right near your adjoining wall. Mains wiring running through walls and floors, faulty household appliances containing electric motors, and physical obstacles like brick walls also play a part in degrading performance.