`fibonacci` is a terrible benchmark and does not use the GC anyway. The difference that you're seeing for that one might be related to the different exception handling implementations. You can try `nim cpp` for a "zero overhead" exception handling implementation that works with ORC.
The other benchmark ... I don't know. It's naive code and you're at the mercy of Nim's optimizer which got better after the release of 2.0 but in general is pretty unpredictable. :-)
