This FAQ may help with the slowness, depending on exactly what the problem is:
[Crash, exit or high CPU when viewing certain directories)
If you need to use Explorer it's still there, just run Explorer.exe from the Start Menu.
You can also make a button in Opus which launches Explorer showing the same folder as the Opus window:
Explorer /e, {sourcepath$}Of course, you can also quickly disable Explorer Replacement mode via Preferences if you need to use Explorer a lot for a while for some reason. Opus will still be available, it just won't be launched when you double-click folders in other programs or on the Desktop.