Polls – How to collect user task processing times?

I have to do a cost-benefit analysis for an ongoing project. To obtain financing, I have to prove that the task to be repaired takes a lot of time for the users. We will only need to test about 20 users, but we will need multiple responses from each user.

Should I just send something to all users where, for example, they click "Start" and then "Finish" and record the times or the time between clicks?

What other tools, strategies or methods are available to time users in this way?