Running a wifi speed-test is a great first step in troubleshooting an issue!
1. On your computer. Go to Speedtest.net and click Go to run the test. It will run a few tests and generate a results page.
2. Take a screenshot of this and send it over to us.
3. Try this in different locations if you're working from home for example. For example, one speed-test ran from where you normally work. Run another right next to where your wifi router is located.
NOTE: It can also be a good idea to compare a speed-test when tethering to a mobile hotspot rather than your wifi. Have a look at this guide from Apple on connecting to a hotspot:
https://support.apple.com/en-gb/HT204023