Under 64-bit Windows 7, I noticed the latest version of Acrobat Pro X (V10.1.2) becomes unresponsive for about 4-5 minutes after opening a PDF file.  Then everything is fine.  I found references to this problem being related to protected mode being used when opening PDF documents.  In the program's preferences, there's an Enhanced Security section.  When I disabled enhanced security, PDF documents started opening quickly.

Then I went back and turned enhanced security back on and added folder paths to the Privileged Locations you can specify as part of enhanced security and was able to open files from these locations without the delays.  Testing is not conclusive since I have been unable to make it go from fast to slow predictably.  However, turning off enhanced security was conclusive.