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When does the slowdown happen? What is on-screen at the time?
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Make sure Windows 10 is fully patched. A bug in some versions caused this with Opus and some other programs.
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Make sure nothing in your Opus config is trying to talk to a network machine which doesn't exist. That can include things like buttons on toolbars where the icons are on a network path which is unreachable. (Things like that will normally matter more than more obvious things like having a network folder in the default lister when it opens, which is usually fine, although worth avoiding when testing for problems.)
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You could try using Process Monitor to see what Opus is accessing during startup.
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You could try manually generating a Task Manager dump during the slow-down, and sending it us.