Bottomline is that wifi extenders don't have very good bandwidth, especially in wifi congested areas. This is because they have to use the same atenna's to receiver and also transmit back to the main router. This effectly cuts any available bandwdith in half and introduces latency and potential bottlenecks. They may advertise 300mbps or whatever but in my experience helping people with wifi problems in their homes, they get more like 5-80mbps of actual sustained transfer speed depending on the amount of walls and distance. That's bits, not bytes, it's really pathetic. On a connection like that, when someone is streaming netflix off the same repeater, you're pretty much saturating your connection and ping will spike like crazy, and you'll get dropped packets.
For most people, I try to run ethernet up through their attic or basement to the other side of the house, and added an access point with the same SSID name and password. In one extreme case where that wasn't possible, I had them buy some burial capable ethernet cable and we ran it outside their house to the other side by digging a little 1foot deep trench.
But in many cases, I was able to find both ends of a Coaxial cable, with a multimeter by shorting 1 end, already installed in the house. Disconnect the coax from the main splitter, and install MOCA adapters on both ends. This setup works extremely well without running any cables, and gives you 90% of full sustained gigabit speed and is very reliable.
If none of those are an option because you live in an apartment and the MOCA adapters don't work due to not being able to access cable splitters, then powerline extenders are usally the way to go.