Something you could try. When the open dialog finally arrives, at the top of the left pane is a drop down box for history, drawings, templates, custom. Is your set to History. If so, use the vertical slider in the left pane to count how many history items it looks for, the default I think is 20, but can be set as desired by right clicking in the pane, and choosing Define history length. too high a value could slow things down.
As a test, if you don't mind losing the history thumbnails until you have opened the files in TC again, you could clear the history (available in the right click menu as above) in case something in the history log or something the log is trying to find, is causing a problem, . The log is actually a text file probably in (assuming windows 10), C:\Users\User Name\AppData\Roaming\IMSIDesign\TurboCAD 2018\Deluxe x64\History.
I have experienced slow file opening dialog, up to 30 seconds. But it appears to have cleared up, this could be one of two things, either something in the history log got knocked off the bottom of it, (mine is set at 20, so saving another file, knocks one of the older files off the list). Or. Windows 10 sent a weekly update which cured the problem. I don't know which of these is true.