Steve,
I know there is an option to exclude folder/files in the Windows 8 - Windows Defender from scans...but I don't believe that this setting applies to the so-called real-time protection or on-access scanning.
In fact, I added the shared folders to the exclusion list...and still had the issue. Turning off real-time protection did mitigate the issue...but I still saw intermittent stalls.
Only after I disabled Shared Folders altogether did the problem completely disappear.
Dr G