run a command, capture output, what form do you want?

[Gribouillis]

the question is, what way would you like to have your code get this data?

The way you describe the problem, it it clear that the subprocess' stdout becomes a source of data that must be read as soon as it is available, so the following should work

1. Create an instance of selectors.DefaultSelector and register EVENT_READ for the subprocess' stdout in this instance.
   
2. Call the select() method, which returns a real time sequence of when data has been produced by the subprocess. For each event in the sequence, invoke a callback that reads in the subprocess stdout and attach a time information to the data by calling for example time.perf_counter().
   
3. If the program needs to do something else at the same time, create a thread devoted to reading the subprocess' output. Clearly the thread that receives the data must not do anything else, otherwise the time information will be damaged.