I see. Just removed the lazyness and got ~800ms.

(defn generate-tree [[h & t :as coll]]
  (if (seq coll)
    (let [lr (generate-tree t)] [h lr lr])
    nil))

(defn to-list [[v l r :as node]]
  (if-not (nil? node)
    (into
     (to-list l)
     (conj (to-list r) v))
    []))

On Monday, July 23, 2012 11:20:50 PM UTC+3, tbc++ wrote:
>
> > Thanks. But I don't do any number crunching here - just a huge structure 
> > creation in memory. I can't get why removing the 'lazy-seq' wrapper from 
> the 
> > second 'concat' argument make things 10x times slower. 
>
> Lazy-seqs require the allocation of a LazySeq object. Due to the lazy 
> nature of this structure, we have to protect against multiple 
> evaluation. This means that certain methods have to be synchronized: 
> (
> https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/LazySeq.java).
>  
>
> All that makes it quite a bit slower than a simple Iterator. 
>
>
> Timothy 
>

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