string-for-each vs. for-each+string->list performance

[Aleix Conchillo Flaqué]

Hi,

in the latest guile-json, 4.1.0. I changed some code to use
for-each+string->list. The performance seemed nice and I released it.

Christopher Lam pointed out that I could have used string-for-each instead.
I made the change but the performance degraded a lot:

string-for-each:

scheme@(json parser)> ,t (->bool (scm->json-string json))
$19 = #t
;; 17.909537s real time, 18.063382s run time.  0.207281s spent in GC.

vs

for-each + string->list:

scheme@(json parser)> ,t (->bool (scm->json-string json))
$20 = #t
;; 2.998381s real time, 3.319349s run time.  0.471969s spent in GC.

string-for-each is implemented in scheme here, if Im not wrong:

http://git.savannah.gnu.org/cgit/guile.git/tree/module/rnrs/base.scm#n89

string->list and for-each would use C.

Is that huge gap expected?

This is with Guile 3.0.2.

Best,

Aleix