Wintendos must go to the PDC Emulator which, in AD exists in only one place... presumably at a head office location connected by WAN. That would explain some of the slowness. The remaining unexplained slowness is the client timing out on API calls.
My guess is that the network guys made some change to AD, either in FSMO roles or permissions, or they applied a hotfix that is causing the greif.
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Give a man a fish and he will be back for more. Slap him with a fish and he will go away forever.
