I did a similar test on Windows: test_io takes ~19.1 sec with and without timeout. If I look closer, the timeout overhead (call a thread per function call) is 112 µs per function call. The average duration of a test is 48 ms (47,750 µs), so the overhead is 0,2%. I am not sure of these numbers (it is difficult to get reliable numbers, especially in a VM), but I'm sure that the overflow is very low.