If your Mac’s Wi-Fi or even a cable Ethernet connection has suddenly become slow, it could just be that your modem needs a quick restart. To restart your modem to fix internet problems, you must ideally shut off your Mac as well, so it can freshly connect to your modem once you have started it up.
So, power down your Mac and then power down your modem. Once your modem is off, wait for at least 15 seconds before you turn it back on. The 15 second wait is essential as your modem has a small battery that has the ability to save and use your existing settings for at least 15 seconds, in case of a very small power cut that lasts just a few seconds.
After 15 seconds, your modem will use default settings and this is what you need to fix your internet problems. Please note that restarting your modem after a 15 second wait is not the same as resetting the modem. Resetting the modem by using a small pin to poke the reset button will often mean that you will need your cable operator’s support, as your modem will be reconfigured with a lot of settings changes. So, do that only when you have their help. Otherwise, a power down and restart is more than enough.
Once you have restarted, you can see if your Mac connects to your Wi-Fi connection properly. Use an internet speed test site like Speedtest.net to check your download and upload speeds. If they appear to be normal, your Wi-Fi is back to normal. If they are still buggy, what you can also try doing is to go to Wi-Fi settings and then make your Mac “Forget” your existing Wi-Fi connection, then rename your Wi-Fi connection to a new name and then reconnect again. If internet on your Mac is very slow and you are breaking your head on why is my mac so slow all of a sudden, when connecting to the net, this fix might end your troubles.