I have noticed (for a long time now) that when adding NPCs from Encounters to the CT, there is a long delay. Here is what I posted on Discord.
https://discord.com/channels/2745828...12046112792696
with attached GIFs demonstrating (kinda).
This was tested in a new campaign and and old one. Its the same. It took over 1 minute to add 15 NPCs. Then when you click on the NPCs link in the CT, sometimes it takes a similar amount of time to bring up that NPC window. I know that SWADE makes duplicates of each NPC added to the CT, so allow them to be customised individually (like adding weapons to them on the CT), but I also test this in D&D 5e and its a lot quicker (65 NPCs in 30 seconds).
If I click each NPC and open their window up, it speeds stuff up for later on, but its initially super slow. Also on the Combat Tracker there is an option to Remove Effects, that also is very slow. (Remove NPC effects, Remove PC effects).