Which columns do you have displayed? Try turning off all columns except Name and see if it is still slow.
File formats which use structured storage can trigger deep virus scans which cause delays when the files are opened (which Opus will do to populate certain columns), although it'd be odd (but not impossible) if they're only triggered by details and not thumbnails mode.
If you have any label-filters set up which use tags or other metadata, try disabling them.
Also, try using ShellExView to disable shell extensions which may be involved, particular ones to do with Solidworks. (After disabling them, fully exit Opus, then re-open it.)