Hi @florian_Gautier,
the time difference is definitely not due to a difference in compile/interpret times. It might have to do with Slow down reading many TFiles (@pcanal might be able to comment).
You can check where the program spends time e.g. with perf record and perf report, or with the poor man’s profiler, which works very well when there is a single callstack that occupies most of the time, as it is probably the case here.
Cheers,
Enrico