https://bugs.winehq.org/show_bug.cgi?id=54769
--- Comment #5 from Rafał Mużyło galtgendo@o2.pl --- See https://github.com/dnSpyEx/dnSpy - a fork, that's still being updated; original has ceased being a few years ago.
Also, your wine-mono installation must be weird (augmented by upstream dotnet ?) for the framework version to not at least complain.
That delay seems indeed fixed in 8.21 - well, maybe earlier, I didn't need to use dnSpy lately.
As for 64bit, that problem had been confirmed (either in a different bug here or in the chat) as a known problem with 64bit implementation of some function in wine-mono. (after checking the logs, it was bug 52396 - supposedly fixed now).
Yet, now it seemingly hangs after showing the window in the decompiling stage...and it happens in 32bit and 64bit both. It eventually gets through, but it takes a bit over a minute. So, even though old problem seems fixed, there's still more to do there. It might still be a regression, if a different one.