Unfortunately there's no built-in mechanism for layout suspending because only the main(UI) thread can change the layout (add shapes to the visual tree). There's a possible workaround - you could try to add the connections in sets using a dispatcher timer. This way the UI won't be frozen but the actual operation will take more time depending on your interval and set size.
I've attached a sample project demonstrating this approach so could you please examine it and if you have more questions feel free to ask.
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