transfert with dolphin very slow

[Raph]

When I trasfert some or a file by dolphin to an another computer with sftp, the transfert is very slow and that for example 1 big file .zip by dolphin is ~7-8MB/s and by scp is 30-40MB/s..

Description: Ubuntu 11.04
Release: 11.04

kio-ftps:
  Installé : (aucun)
  Candidat : 0.2+dfsg-2build1
 Table de version :
     0.2+dfsg-2build1 0
        500 http://ch.archive.ubuntu.com/ubuntu/ natty/universe amd64 Packages

ProblemType: Bug
DistroRelease: Ubuntu 11.04
Package: kio-ftps (not installed)
ProcVersionSignature: Ubuntu 2.6.38-11.48-generic 2.6.38.8
Uname: Linux 2.6.38-11-generic x86_64
NonfreeKernelModules: fglrx
Architecture: amd64
Date: Sat Aug 27 17:25:23 2011
EcryptfsInUse: Yes
InstallationMedia: Kubuntu 11.04 "Natty Narwhal" - Release amd64 (20110427)
ProcEnviron:
 LANGUAGE=fr_CH:fr
 LANG=fr_CH.UTF-8
 SHELL=/bin/bash
SourcePackage: kio-ftps
UpgradeStatus: No upgrade log present (probably fresh install)