file transfer using SCP protocol with winscp (5.7.4) is really slow only under debian 8.1 jessie. the cpu usage from sshd goes to 100% and the speed breaks down after few seconds to 500~ kb/s. i found out, disabling the feature "optimizing buffer size of connection" in winscp fixes the problem. cpu usage goes to a normal level and the speed goes up to 90 mbit.
FYI: i was using before debian 7 wheezy with the same winscp version and the same sshd config it is working fine.
so it looks like winscp and the feature has a problem with the newest sshd version.
debian 8.1 jessi is using: OpenSSH_6.7p1 Debian-5, OpenSSL 1.0.1k 8 Jan 2015