There's a myriad of ways to deploy wifi, and loads of different versions of the specifications kicking about.
Probably the most important thing to understand is that in a wifi network only one person can talk at a time. So let's say you have just the two computers, and a router, that's three devices, all of which need to talk, and only one can talk at once. Your two computers don't talk to each other though, they talk through the router. So Sierra says to Bob (your router) "here's 1/100,000th of that file, pass it to Mojave". Bob then gives the file portion to Mojave. Mojave then tells Bob to tell Sierra than it's arrived. Bob tells Sierra. Sierra then tells Bob to send part 2/100,000 to Mojave. Bob tells Mojave it has a piece of file and sends it over. Mojave confirms it got it back to Bob. Bob tells Sierra.
That's how it works ideally. It's cumbersome. In real life, it's worse, because at any time when Bob is giving the file to Mojave, then Sierra can say "Oh hey Bob, I've got another piece" and then Bob and Mojave have to stop, every has to shut up, then Bob tries to send the piece to Mojave again, and Sierra will again try to tell Bob that it has more file. And worse, let's say you have a phone with wifi. That's also trying to squeeze in it's downloads of CNN or whatever. And a wireless printer. And whatever else you have.
So ideally you don't want to transfer files between wireless devices on the same network. I have two wireless networks. Three I suppose, including a virtual one. I also have a wired network in the back of my wifi router.
So again, without knowing about about your network it's difficult to help, but hopefully the above makes at least a little sense. If both of your device are wifi, then put them on different wifi networks (one on 2.4GHz and one on 5GHz) or plug one of them into the router directly. Again, don't really know anything about your setup, but that will almost certainly help if you can do one of those two things. And AFP instead of SMB.
wired? Wifi? Modems? 