They don't compare contents (only the duplicate finder does that).
(It'd usually take the same amount of time to just overwrite the files without checking, if the whole files have to be read and compared.)
You may find that something triggers offline files to be downloaded though (e.g. if certain file display columns are turned on), so I would be careful of that.
With a huge number of files, using Copy is fine. Using the interactive Sync tool may not work that well as it can get very slow with that many files (depending on the exact situation). (That's something we might improve in the future, of course. And you may find it's already OK for your use-case, depending on the exact details.)
If you want an interactive sync with a large number of files/folders, where you need to see what's going to happen and modify what will happen for individual files and folders before committing the change, a dedicated diff/merge tool might be worth looking at. I use and like Beyond Compare personally, but there are also free tools like WinMerge which are pretty good too. They can be tightly integrated into Opus so you just push a button to launch them against the left and right folder: