The fastest way is to connect the two computers using Thunderbolt. This will give you 10 Gbps of bandwidth, which is 10 times what you would get from the ethernet port on the 2011 MacBook Pro.
Note that if you have only one drive in your old MacBook Pro, you'll technically be limited by the speed of that drive when transferring. I.e. if you have a 6 Gbps drive, you'll only be able to transfer at that speed - which will probably use approx. 7 Gbps of Thunderbolt bandwidth. If you've got the two drive mod for the old MacBook Pro, you'll theoretically be able to use the full bandwidth of the Thunderbolt connection.
You'll need a Thunderbolt to Thunderbolt cable and a Thunderbolt 2 to Thunderbolt 3 adapter cable to connect the two Macs. As Thunderbolt 2 is backwards compatible with the original Thunderbolt, this will allow you to link the two Macs together. Then boot up the old Mac in Target Disk Mode (hold down the T key while booting) - and you can use it as a data source in Migration Assistant.
If you haven't got those cables lying around, chances are that it is probably faster to just use Ethernet rather than waiting for cables to be delivered. WiFi is definitely the slowest approach here.