This sounds like the issue has been isolated down to one device. I am gonig to provide a few links that includes the minimum network speeds, supported devices, and supported browsers.
Here are those articles as promised:
- Which SlingPlayer software works with my Slingbox?
- What browsers and systems are supported by Watch on Slingbox.com?
If this is not the cause, have you added a new ant-virus program? Anti-virus programs may cause the SlingPlayer plug-in to run rather slow. I would recommend disabling this to see if the issue continues.
This article below will explain a little further.
Sling Media Moderation Team