All I can observe, in a worst-case scenario with a setup that is so slow it takes half an hour to open the project, is it still only takes 1 second to open the dialog. It has to fill it with several thousand items in such a large project, so it doesn't seem unreasonable it would take that long. It is probably limited on the performance of Windows' implementation of the controls that are being filled with a lot of content.
If the performance issue was in Construct 2's code, I should have been able to observe it in the setup I previously used. Since project loading is in Construct 2's code, and that was all unoptimized and running very slowly, as expected I could observe project loading taking an extremely long time. However when it comes to opening the dialogs, I could not observe anything as bad as anyone is reporting here (i.e. 6-7 seconds). That suggests the slowdown is not in Construct 2's code. As further evidence to that there is apparently variance across Windows versions and possible correlation with Windows updates, which further suggests that opening the dialog is bottlenecked on the Windows internal implementation of filling up controls with lots of data.
Assuming this is the root cause here, unfortunately we cannot take responsibility for the performance of the internals of the Windows operating system. If that is the case, it's not clear what we can do about it. What would you propose we do?