I had a different plan to attack this problem:

Installer squeak
 	project: 'inbox';
 	install: 'Multilingual-ul.102.mcz';
 	install: 'Collections-ul.336.mcz'

Benchmark:

((1 to: 10) detectSum: [ :run |
 	|converter|
 	converter := UTF8TextConverter new.
 	[ 1 to: 50000 do: [ :index |
 		'abcaskjdhfáasiugbvsipruvnaséipvunasivunapiívunasieunó'
 			convertToWithConverter: converter ] ] timeToRun ]) / 10.0
"old ==> 355.6 new ==> 273.0"


Levente

On Sat, 13 Mar 2010, commits at source.squeak.org wrote:

> Andreas Raab uploaded a new version of Collections to project The Trunk:
> http://source.squeak.org/trunk/Collections-HenrikSperreJohansen.335.mcz
>
> ==================== Summary ====================
>
> Name: Collections-HenrikSperreJohansen.335
> Author: HenrikSperreJohansen
> Time: 12 March 2010, 3:38:49.316 pm
> UUID: 8b8f9b48-feb7-7d4d-9458-dcd334ad6e81
> Ancestors: Collections-ar.334
>
> Faster String>>convertFromWithConverter: from Pharo.
>
> Useful in f.ex. asVMPathName.
>
> Test:
>
> [|converter|
> 	converter := UTF8TextConverter new.
> 	1 to: 50000 do: [:ix | 'abcćřĺaskjdhfasiugbvsipruvnasipvunasivunapivunasieun' convertToWithConverter: converter]] timeToRun
>
> =============== Diff against Collections-ar.334 ===============
>
> Item was changed:
>  ----- Method: String>>convertToWithConverter: (in category 'converting') -----
> + convertToWithConverter: converter
> + 	converter
> + 		ifNil: [^ self].
> + 	^ String
> + 		new: self size
> + 		streamContents: [:writeStream |
> - convertToWithConverter: converter
> -
> - 	converter ifNil: [ ^self ].
> - 	^String new: self size streamContents: [ :stream |
> - 		| character |
> - 		1 to: self size do: [ :index |
>  			converter
> + 				nextPutAll: self toStream: writeStream;
> + 				emitSequenceToResetStateIfNeededOn: writeStream]!
> - 				nextPut: (self at: index)
> - 				toStream: stream ].
> - 		converter emitSequenceToResetStateIfNeededOn: stream ]!
>
>
>