Uploading speed difference

Hi,
I have used NC for a about 2 years. I found a problem when i upload files:
When i upload files as local user, the speed is about 50M/s;
When i upload files as Ldap user, the speed is about 5M/s;
I deploy NC in local server.This situation appeared in NC 16/17/18.

How can i increase the uploading speed of Ldap users from 5M/s to 50M/s ?

Thanks!