Slow to open progress window when copying many selected files (100,000) on external device

* Create a folder containing 100,000 small files on an external device
* Create a destination folder somewhere
* Display a view of the source files (use List View for higher speed) - may take 10's seconds.
* Press `A` to select all the files then `C` to copy to clipboard.
* In another tab, display a view of the destinations *parent* folder
* Use "Paste into Folder" to start copy.
* Nothing seems to happen for 10's seconds. Then the progress window opens showing "Preparing message". The operation continues normally. The rate of transfer drops for a while but eventually stabilises (Not unexpected as calculation becomes more reliable over time and effect of caching diminishes)