Opus 10.5.4 Windows 7. New off-the-shelf Dell computer. Still doing conversion.
When copying files from harddrive C: to an external (USB3) drive K: the action pauses frequently when looking at progress bars. (Work-work-work-stall-stall-work work-stall). Example during such a pause. 
Seconds later it will start back up. When it works it works well.
Where can I start looking to debug/correct this behavior?
Anecdotally, USB3 copy problems are sometimes the result of mounting a bare USB3 drive in an external dock; the problem being caused by the dock rather than the disk. Another thing worth checking is whether you have the latest BIOS level in your new Dell.
Regards, AB
AB: thanks for the thoughts.
The port is a USB3 -- and the speed statistics seem good.
I'll check the bios.
The timer continues ticking during these pauses, which seem to happen when copying a large file; but mb not gb in size.
Maybe I have too high expectations from all the money spent and am just anxious...
This happens to me too all the time. USB 2.0 stick on USB 2.0 port.
The device probably has a cache that gets filled up quickly and then it won't accept any more data until it has written the cache to disk.
Turning off non-buffered I/O may make things smoother, although the overall copy speed is unlikely to change unless there's something actually wrong with the device/controller/drivers (which does happen sometimes; some of them don't handle non-buffered I/O properly).
Things like real-time anti-virus can also add delays, as can something else (could be part of Opus, could be something else entirely) accessing the same device at the same time, if it's a device/controller/driver that doesn't handle parallel access very well.
On Win 8 the cache for ext. drives (HDD, Stick,...) is deactivated by default, so that you can unplug the device w/o a safety remove. You can try to activate it for each device, but it should not make remarkable differences in speed or when copying files in DO, except the "unbuffering time" at the end (copy-dialogue is at 100%, but it still stays some seconds). Most times such a behavour is BIOS- or driver-related.
The device itself may have its own cache, however, even if it is mounted as removable or with the system writeback cache disabled in Windows.
If it's a HDD then "may" = "will".
Sure, but nevertheless Windows also heavily uses its own cache for HDD's and for ext. HDD's it is deactivated by default (Win 8).
Is that relevant in any way to this thread?
Just answered your objection and I originally answered to jsys (using an USB-Stick and Win8).