There are many possible reasons for that, ranging from data caching (if you copy a file with one program, then again with another, the data may be cached and much faster), to buffer sizes and non-buffered-IO settings (you can adjust those via Preferences if you want to experiment; type buffer into the Preferences filter to find the appropriate options), to peculiarities with hardware, drivers or antivirus in how they respond to different programs doing things in slightly different ways (sometimes solved by updating the firmware/drivers/software of those things; sometimes just because they were only really tested against Explorer).
With video files, problems may also occur if Opus is configured to display any columns (or thumbnails) that extract information from video files and that extraction process ends up reading a lot of data. (Exactly how it works depends on your video codecs and sometimes also shell extensions.)
When copying lots of small files, the overhead of copying file attributes, descriptions, permissions, and other metadata can be very significant, and Explorer does not copy the same details for each file that Opus copies by default. (That can be changed via Preferences as well.)
There's no definite answer, and we couldn't tell you without looking at your actual machine, so it may take some experimentation or investigation (e.g. using Process Monitor to see how the file or other files in the same directory are being accessed while the file is being copied).
All we can say is that on our own machines, Opus copies as fast as the hardware will allow, and typically as fast as Explorer or anything else in a fair test (both configured to copy the same metadata, none of the files already cached in memory by a previous copy).