(defn using-xform--improved [chunks]
  (transduce (comp (halt-when error?)
                   cat
                   (x/by-key :a x/count))
             conj
             {}
             chunks))

(using-xform--improved chunks)
=> {0 3330, 1 3330, 2 3330} ;; Still works

(using-xform--improved mini-chunks-with-error)
=> {:error "Something terrible has happened!"} ;; Better!

(criterium/bench (using-xform--improved chunks))
"
Evaluation count              : 156300 in 60 samples of 2605 calls.
Execution time mean           : 405.974099 µs
Execution time std-deviation  : 21.578325 µs
Execution time lower quantile : 377.589901 µs ( 2.5%)
Execution time upper quantile : 448.218002 µs (97.5%)
Overhead used                 : 1.481300 ns

Found 1 outliers in 60 samples (1.6667 %)
	low-severe	 1 (1.6667 %)
Variance from outliers : 38.5251 % Variance is moderately inflated by outliers
"