I have noticed now many times that i can copy a file from one hd to another and sometimes it's incredibly slow like 3mb/s and when i try to copy the same file again just seconds after finishing the copying it's just as fast as it should be about 55-60mb/s.
There is no other disk access going on at the time either i have checked with Sysinternals Filemon that could interfere with the file transfer.
This problem happens at random. There is never a problem when using Explorer to copy files.
If it's slow the first time and fast the second, with the same file, then it could be because the second time the file is cached in memory.
Or anti-virus scanning the file the first time you read from it, but remembering it has scanned afterwards.
Disk fragmentation in the source or the destination could account for slowness some times.
As could background tasks kicking in.
None of which would explain why the same doesn't happen in Explorer, but have you done a lot of tests in Explorer or only tested it after seeing slowness in Opus, which could be attributed to one of the above things?
One difference between Opus and Explorer is that Opus's progress window has more going on, especially if you are using the "full" progress window with the speed counters and time estimates. In the past this has triggered problems with some bad motherboard drivers (VIA I think; not sure) because the extra graphics updates going over the bus were conflicting with the IDE data also going over the bus. The person who had that problem solved it by updating motherboard drivers.
You may also want to try changing Opus's copy buffer size to see if that affects things. It's been known to with some devices. You can change the buffer size in Preferences - File Operations - Copying Files.