Without this optimisation, `cabal info somethingnonexisting` results in

     960,397,120 bytes allocated in the heap
     739,652,560 bytes copied during GC
      67,757,128 bytes maximum residency (24 sample(s))
       2,234,096 bytes maximum slop
             147 MB total memory in use (0 MB lost due to fragmentation)

with this optimisation:

   1,000,825,744 bytes allocated in the heap
     656,112,432 bytes copied during GC
      44,476,616 bytes maximum residency (24 sample(s))
       2,302,864 bytes maximum slop
             109 MB total memory in use (0 MB lost due to fragmentation)

So the total memory in use is significantly lower. The total runtime is
also slightly reduced, from

  INIT    time    0.001s  (  0.001s elapsed)
  MUT     time    0.683s  (  1.050s elapsed)
  GC      time    0.946s  (  0.946s elapsed)
  EXIT    time    0.005s  (  0.005s elapsed)
  Total   time    1.637s  (  2.002s elapsed)

to

  INIT    time    0.001s  (  0.001s elapsed)
  MUT     time    0.664s  (  0.988s elapsed)
  GC      time    0.797s  (  0.797s elapsed)
  EXIT    time    0.004s  (  0.004s elapsed)
  Total   time    1.467s  (  1.789s elapsed)


Note that there's currently ~80k cache entries, but only ~10k unique package names
and ~6k unique versions. So hash-consing helps reduce the amount of heap objects 
for both value types by one order of magnitude, which among other benefits also
reduces GC overhead.