Concurrent data structures are rarely faster than "lock + non-concurrent data structure" and if you're putting constructs like par_iter() in a lot of places, there's a good chance you would be better off with the "dumb" pattern than the concurrent data structure.
The same goes with Arc - if you're using it a lot there's a good chance that code with a lot of Arcs is slower than equivalent GC-ed code.
The same goes with Arc - if you're using it a lot there's a good chance that code with a lot of Arcs is slower than equivalent GC-ed code.