Migrating to doubles and unchecked-math makes everything faster...for reasons....

[joinr]

Mixing the long (in clojure) index with the double mult is hitting a slow path and invoking clojure.lang.Numbers

(defn calc-pi-leibniz2
  "Eliminate mixing of long/double to avoid clojure.numbers invocations."
  ^double
  [^long rounds]
  (let [end (+ 2 rounds)]
    (loop [i 2.0 x 1.0 pi 1.0]
      (if (== i end)
        (* 4.0 pi)
        (let [x (- x)]
          (recur (inc i) x (+ pi (/ x (dec (* 2 i))))))))))

leibniz=> (c/quick-bench (calc-pi-leibniz rounds))
Evaluation count : 6 in 6 samples of 1 calls.
             Execution time mean : 575.352216 ms
    Execution time std-deviation : 10.070268 ms
   Execution time lower quantile : 566.210399 ms ( 2.5%)
   Execution time upper quantile : 588.772187 ms (97.5%)
                   Overhead used : 1.884700 ns
nil
leibniz=> (c/quick-bench (calc-pi-leibniz2 rounds))
Evaluation count : 6 in 6 samples of 1 calls.
             Execution time mean : 158.509049 ms
    Execution time std-deviation : 759.113165 ╡s
   Execution time lower quantile : 157.234899 ms ( 2.5%)
   Execution time upper quantile : 159.205374 ms (97.5%)
                   Overhead used : 1.884700 ns
nil