HughesNet is stable enough to handle Amazon Prime, as I've watched it myself.
What you're probably experiencing is a corrupted cached DNS entry, not an issue with lag time. Amazon likely uses a different path to it's streaming content than it's menuing application and the images it uses, and that path has been somehow corrupted. I've experienced this myself at times.
To correct this, you may want to:
1. Unpower your modem by removing the cord from the power brick - this will automatically kill any cached DNS entries.
2. While the modem is off, similarly remove the power input to your Roku - this will kill any cached entries in the Roku.
3. You may want to optionally unpower or restart anything else connected to the wifi to ensure those devices aren't similarly harboring any cached DNS entries that are corrupt.
4. Re-apply power to the modem and wait a few minutes until the wifi reappears.
5. When the modem is fully operational, restart your Roku (and any other devices).