Why is my program so much slower in Nim than in Rust?

[jrfondren]

    ========   =======   ==================================================
     Run ms     Speed     Flags
    ========   =======   ==================================================
      157.5     2.02x     -d:release
      113.7     1.46x     -d:danger
      158.0     2.03x     -d:release --floatChecks:off
      123.7     1.59x     -d:release --overflowChecks:off
      145.1     1.86x     -d:release --boundChecks:off
      112.8     1.45x     -d:release --overflowChecks:off --boundChecks:off
       77.9        1x     -d:danger --cc:clang
      197.8     2.54x     -d:release --gc:arc
      177.9     2.28x     -d:danger --gc:arc
       86.8     1.11x     -d:danger --cc:clang --gc:arc
    ========   =======   ==================================================

Conclusion: --gc:arc does this no favors. Integer checks for all the loops are 
what slows -d:release down. LLVM beats the pants off gcc for this benchmark, 
which might explain Rust's performance.