What is the fastest way to transfer 200GB data from 2014 Macbook Air to 2016 Macbook Pro What is the fastest way to transfer 200GB data (~ 2 million files) from 2014 Macbook Air to 2016 Macbook Pro and how long will it take?
The fastest method is to use Target Disk Mode using the Thunderbolt ports on the machines.

You can refer to this guide from Apple:

https://support.apple.com/en-ug/HT201462

Note that because your MacBook Pro features Thunderbolt 3, you'll need to use an adapter to interface with the older Thunderbolt port on the MacBook Air. The directions on getting the correct adapter is also in that link.

Regarding how long time it will take is hard to calculate precisely - it involves many factors such as the type of SSD in your MacBook Air, the file system used, what else the computer is doing at the same time, etc.

An estimate would be approx. 7-10 minutes.