I have encountered at least two server 2008 R2 machines that were recently built that exhibit identical behavior. When using explorer to copy a folder of files from or to the affected host, any file that is +R (read only) attribute flagged causes the copy to hang for 30 seconds or more before proceeding.
If I enter the folder in command prompt and run attrib -s -h -r * /S, this strips the +R off all files in the tree and they will copy instantly, without any timeout delays.
No errors are being logged. I am stumped.