Thank you for reporting a bug to Unity.
From our investigation, we found there is no significant change in the actual Windows Editor shutdown time, w.r.t. 6.3 release.
The change identified in this ticket comes from the fact that the MainWindow now closes immediately when the 'X' button is clicked, which occurs before the main Editor shutdown operation. In previous versions, the MainWindow would close after the main Shutdown operation had completed, causing the MainWindow to "hang" for 1-2 seconds after the clicking 'X'.
In some scenarios, this makes the Editor feel slightly more performant, since closing the MainWindow responds immediately while the remaining shutdown runs in the background. However, in scenarios that require the Editor to fully exit, e.g. re-opening the project, this may feel like the Editor performance has regressed.
In actuality, the total shutdown time has remained about the same.