how to calculate actual time taken to download a big file in rest API testing using mocha & chai?

scenario is: - I have to select a big file (say 1gb or 5gb) - then click on download button - get 200 response - then get final time taken to download a file

When i am calling download api then file is getting downloaded and I get 200 response as well. even get response time as well which is 80-120ms.

But my actual requirement is when file fully downloaded in my system then get real time taken to download..