Some file selection dialog (and explorer) issues come down to the anti-virus. I've had folders on an SSD that took a minute (60 seconds!) to load. After I added it to an exclusion list in Defender it loaded in a second.
Another one that can be slow with file dialogs is that sometimes (maybe it has been fixed now) it will try to query whether a networked drive is around on another computer. If it isn't then the call to it can be blocking your file UI.
A third problem I've noticed with file selection dialogs and explorer is that the My Computer 'folder' that contains your disks takes a long time to load. Much longer than any sub-folders on any of the drives.
I think the problem is largely with explorer.exe. If I browse those folders in a web browser the experience is snappy.