Xcode 12.5.1 seems to have fixed the issue for me: the first build and run took 30+seconds to attach, however the second and third build and run attached after around 3 seconds.

Anyone found any workarounds other than scheme duplication?

I have filed a request for this here: FB11810918

Today Apple appears to have resolved the issue server side - I was able to upload my archive without recompiling from yesterday and it got accepted.