I have a fairly decent laptop provided by my employer and I'm lucky enough to have a strong, reliable internet connection. Not everyone does. I'll say that again for the Bro's at the back:
Not all of your users will have a top-spec device and good wifi like you do.
In the past I've been guilty of assuming that everyone has a comparable experience to myself.
This leads to Bad Habits™️.
Test on spotty wifi. Try using the site on a train or in a coffee shop or using the City's free wifi (if you have it where you are). This is how your user will experience your site - not via 100Gb Ethernet cables.
Test on budget devices - find a few common mid-priced devices (you could probably get them from eBay) and test on those instead.
If you're feeling extra spicy - make sure at least one of those devices has a cracked screen for a truly authentic browsing experience 😜