That article makes a claim and doesn't back it up. While they can say it isn't computationally expensive COMPARED to 10 years ago, computationally expensive is relative. I believe their assertion is based on the fact that if you are using HTTP(S) in general for communication that adding SSL/TLS on top of that communication mechanism isn't going to destroy the performance of your application. I would rather see some profiling of an example first before believing that though.
We need to understand the type of application you are developing first. Is this an application that will be transporting data through public networks? If it is through public networks will it require a VPN to access thus potentially providing encryption at the VPN layer?
How many HTTP operations per second are you expecting?
You will need to run performance tests to determine if SSL/TLS adds any overhead to your application. Build your application to handle both.
What are the Use Cases for allowing someone to pick HTTPS over HTTP? If all the data is sensitive and some users would want HTTPS, I would question even allowing HTTP communication in the system.