Which directories does Process Monitor show being accessed?
If you don't need your file collections, go to coll://, select all the collections and delete them. That will normally reduce the (usually harmless) noise of file/folder access when Opus starts. Note: Select the collections themselves (so just the collections are removed), not the things inside them (doing that would delete the things themselves).
Does it happen regardless of which directory Opus is viewing?
(i.e. Open just one window, with just one tab, pointing at an empty directory. The best way would be to create an empty dir on your desktop and then double-click it, so that no other directory gets loaded first. Does it still happen then? If it does still happen, see if turning off the folder tree helps. If it doesn't, see Crash, exit or high CPU usage when viewing certain directories for suggestions.)
It's also worth checking if Windows Explorer triggers similar activity when viewing the same folder.