As other answers say, WiFi is a local problem.
It's the internet equivalent of a cordless home phone, a short range wireless link so you don't have to trail a cable from the router.
You need to also be aware that WiFi is based on decades old frequency allocations and the main 2.4GHz band used is massively overloaded.
Although it has 11 (or 13) channels, a single 11g WiFi signal uses a block of four numbered channels - you can only have three separate systems in range using channels 1, 6 & 11 without interference.
With the newer, "faster" 11n systems, those often use 8 channels so it's impossible for even two to work at the same time within a few hundred yards without interference and speed loss.
In many cities, there are so many systems in range of each other than all slow to a crawl.
Plus many other things such as wireless security cameras, baby monitors, microwave ovens and some cordless phones use the same frequencies. They do not show up on computers but the signals still cause interference and mess up WiFi still more.
Add wifi "range extenders" that re-transmit everything they pick up - and add to the congestion in the process - and it's often a miracle it works at all, never mind with any speed or a reliable connection...
WiFi should alway be an absolute last resort, saved for things that have no other possible means of an internet connection such as phones an tablets.
Anything else should be connected by ethernet cable, or if that's not possible then use Homeplug devices that send the signal via the house power wiring rather than radio.
[Electronics & computer designer and programmer for 40+ years].