This is weird, I’d expect the timing should be reversed with your very simple dataset but somewhat involved computations on the samples…
I want to point at something that you might have missed: you update the time_data meter with the lagrange variable, before updating it again with t5-t1. How does your meters work? Does it return some average or the last value? This could influence the printed time_data at the end…
Also, do you really need 20 workers for such a small dataset? Have you tried different values?