This seems to be new to this version (last verstion tested was 1640). After, 1 or 2 hours of usage, the opening of this dialog box gets slower and slower (I use this functio quite a lot). Quiting and relaunching VS2008 repairs the performance. Other VA functions seems to work at the correct speed.
In the Open Files dialog do you have "Show only files in current solution" checkbox checked or unchecked? Having this unchecked is a common cause of this dialog taking a long time to open, as discussed in this forum thread:
Right, if "Show only files in current solution" is checked, the problem does not happen. But I don't see why the problem only happens after using VA during at least 1 or 2 hours.