I didn’t expect this to work 3 years later, on a Mac… but it did!
5 MB/s → 50 MB/s is a substantial improvement! 
For the fellow Mac users, the file you’re looking for is inside the package contents (right click the EGS icon → show package contents) and then /Contents/UE4/EpicGamesLauncher/Config/DefaultEngine.ini
My file had too many things (even had a different [HTTP] tag with different params), I just ignored it and pasted the params from this post at the bottom and it worked!