Our IT department upgraded our McAfee installation just before Christmas to McAfee VirusScan Enterprise Workstation. After they did that Subversion checkouts slowed by a factor of about 20.
So, if you are using McAfee and your checkout performance has plummeted then it is probably McAfee causing the the slow down. Disable on-access scan and it should speed up again.
Use Exclusions
I ran into the same issue with the upgrade to McAfee 8.5i. The on demand scanning killed Subversion performance because it was constantly checking files on every action.
While disabling On-Access scanning will help with the issue, it eliminates a large part of why McAfee is installed in the first place. Instead, set up a scan exclusion for your repository directories and go from there. This gives you the performance you are after in Subversion, while maintaining on-access scanning everywhere else!
That's Good
But if your settings are locked down by your IT department you will have to get them to make the exclusion for you.