In the modern times we are so dependant on the WiFi connection nevermind if we speak for business and personal use.
There are multiple factors that might affect your wifi's performance.
Here is my list to look for:
1) Your ISP's DNS server is slow. That can be solved by using another DNS with better performance. Google has such that might improve your speed dramatically.
2) You're not connected to the best network. Most modern routers are coming with options for multiple wifi networks to be created simultaneusly. Some of those are running on 2.4Ghz and others on 5Ghz. The 5Ghz is way faster but it has smaller coverage and the 2.4Ghz is the other way around. Always test all the options and see what's working best for you.
3) Your ISP might be bugging your connection if they delect heavy usage. If you know what your speed should be you should check it regularly and if this happens address it with them.
4) 2.4Ghz networks are very easily to get full with just several devices connect to it. The solution to this is connecting different devices to different networks.
5) Heavily congested channels - in the router options there are usually 14 channels that your wifi can use. Sometimes in the same aria(like in a big building with multiple appartments) there can be many networks might be running on the same channel. If that happens your wifi won't work at best speeds and you even can get more problems like devices getting disconnected. The only solution for this is switching the channel every once in a while.
6) Sometimes some of your devices can have software issues and to start occupy multiple channels which can be solved by looking of the connection of the device inside the router's options.
7) The technology develops so fast that it is a good idea to change your network cables evey few years. If you are using 10-15 years old Lan cables to get your internet to the router the chances are that you are not using it at full speed. Even better will be if you are able to get internet to your home via Fiber Optics.
8) When you have internet trouble always restart the router first. This is the reason that your Admin is always asking this questiion first. It actually helps to restore the connection in probably 80-90% of the cases.
9) Test and find the best place to put the router on. The placing is very important for the speed you are actually getting in different spots in the house/flat/office. Of course the best way is to not have any obstacles between it and the receivers.
10) Buying an Wifi Extender/Repeater will not increase your speed by itself, but will increase the range of your network and you will be able to use the network at full speed from a bigger distance to the router.
Those are my tips. Please share if you think of anything else.