(require '[net.cgrand.xforms :as x])

(defn using-xforms [chunks]
  (into {}
        (comp (halt-when error?)
              cat
              (x/by-key :a x/count))
        chunks))

(criterium/quick-bench (using-xforms chunks))
"
Evaluation count              : 1578 in 6 samples of 263 calls.
Execution time mean           : 409.277676 µs
Execution time std-deviation  : 42.914476 µs
Execution time lower quantile : 387.744354 µs ( 2.5%)
Execution time upper quantile : 482.870979 µs (97.5%)
Overhead used                 : 1.481300 ns

Found 1 outliers in 6 samples (16.6667 %)
	low-severe	 1 (16.6667 %)
Variance from outliers : 30.7015 % Variance is moderately inflated by outliers
"