I always found Filezilla to almost max out the connection, peaks around 11.2MBps.
Also you can explore the Xbox through Windows. I'm not sure what kind of transfer speeds it gets, but it's pretty painless to set up and very convenient.
Go to "My Computer" (or This Computer for Windows 10) and click either "Map network location" or in Windows 10 right-click the "This Computer" window and choose "Add Network Location."
Click "choose a custom network location" and hit next.
Enter "ftp://xbox : xbox@[your Xbox IP]" - example: "ftp://xbox :
[email protected]" and click next. Also disregard the spaces.
Uncheck anonymous browsing and now you should see in under My Computer.