Hi Richard,
Sorry for the late response to your query.
Where you able to download the latest code for the SRP521? If not, I'd recommend that you do that first and foremost.
The code is available from: http://www.cisco.com/go/srp500 (follow the "Download Software" link.)
Remember to log in as admin/admin to upgrade the firmware on your device.
Can you describe the problem you are seeing with network performance a little more? I presume this is throughput to/from the Internet, rather than between local devices? Are the local devices wired, or wireless?
We have had some reports from a few customers that they see performance reducing as a function of time. This particular issue seems to be mitigated by disabling UPnP - we're invesigating this issue at the moment.
If you are seeing issues, firstly, I would recommend raising a case with Small Business Support Center. To help with diagnose the issue, log in to the SRP521 as admin/admin, then browse to the Administration/Remote Debug page. Click the "Collect Debug Information" button and provide the downloaded file to your support engineer.
Hope this helps,
Andy