We are sorry to hear that you have such issue.
In your situation performance is dependent from different factors such as Amazon Cloud, Internet channel between Amazon server and GroupDocs Server. Please try to move your Amazon S3 storge to another server in another region, which is closer to your users. We didn't have any configurations which can affect on performance from GroupDocs side. Please try to investigate Amazon Documentation for that issue:
If you will have more questions please feel free to contact us.