That has been my experience too. That all the extra work and logic (cause the algorithm is complicated) you need for detecting cycles and trial deletion is so expensive that regular mark&sweep beats it.

But to ask a pointed question, doesn't that mean Nim gets the worst of both worlds? You have both the overhead of updating reference counts and the (relatively long) garbage collection pauses. I guess if the programmer codes in such a way that no cyclic garbage is created it is not a problem because the gc will never be triggered. But how common is that in practice? Does the language make it easy to avoid cycles?