That could probably happen with any API or protocol which sends as fast as it can. You need to configure network QoS settings in Windows to avoid that, unfortunately, as the Windows network stack isn't clever enough to avoid one connection or protocol from starving the other.
That is beyond the scope of this forum, however. The forum is to support Directory Opus, not answer random programming queries. 
Try asking on StackOverflow or SuperUser.
Hi Leo!
Thanks for your time and answer. I actually just wanted to bring this issue to your attention in general, as I'd read your plans about moving to CopyFileEx.
I might have been late to the party though 
Anyway, keep up the good work!
Miki
It's a shame DOpus has given up on this
The effects of this are magnified over FTP, Fastcopy can transfer the same 300MB video file at ~8MiB/s over the internet, whereas DOpus transfers consistently at 800KiB/s
How about an adaptive buffer?
And shame on you @Leo for your attitude towards OP this entire thread
We haven’t given up on it. The change will be released when we’re ready to do so.
(FTP is a very different thing, too.)
It's been four years now. But the problem is much older. I wonder if this will ever be fixed. I have to use Windows Explorer for four years now just to copy a bunch of larger files around.
The change to use FileCopyEx is already in our internal, long-term development builds. We’ve been testing it for a while.
Lots of other things are in the way of releasing it, but we’re finishing them off.
Meanwhile, I’m waiting similar time for completely broken (not just slow on some hardware/drivers) things in other software to be fixed by much larger software vendors. Please cut us some slack.
When i copy files over SMB on my linuxserver.
In explorer i can move files from zfs storage pool to ssd pool on my server at ~400MB/s.
In Directory Opus i get 57MB/s.
Increasing the buffer size to 512MB seems to help with speed a bit, but you get literally no transfer speed showing at all. Just 512MB buffer fill/flush showing. Not at all a solution.
Overall samba performance is horrible in Directory Opus.
Finding this thread that this is broken since several years made my decision to not buy DOpus at all and look elsewhere.
Sad to see. I loved DOpus back in the Amiga days. On PC it have never worked good the times i have tried during the years.
That isn't my experience, but it varies by hardware/OS/config at the other end (and in between, sometimes). Something is ultimately wrong if you're only getting 10th the speed unless the file is written in a very specific way, and that is going to slow down almost everything that saves data to the same server unless it's using CopyFileEx, which is only good for copying a file from one place to another (can't be used to write data from memory, like most programs will do when saving changes, etc.)
In any case, once the CopyFileEx change is released, you should see identical performance in Opus and File Explorer.
Thanks for your reply.
Yes the CopyFileEx change you talked about since 2019? Anyday now?
If something is very wrong, can you point me to some fix for samba performance?
I have all default settings. Just as i have in Total commander that runs full speed no problem.
I have googled this and no solution at all is found in your forums, reddit or anywhere.
Only suggestion mentioned is wait for CopyFileEx (since 2019 and counting) and raise copy_buffer_size.
Some thread from 2006 someone suggested enabling Enable Folder Content Type detection.
I mean you sell a file manager. If it isn't good at managing files, whats the use of the product?
What's the use of continuing to complain about this when you already know what the answer will be?
I understand people encountering this for the first time asking questions.
Only you can decide whether, for you, other features outweigh this problem, but there's little, if anything, to be gained by people already knowing what the situation is continuing to complain about it.
I've read somewhere, that the developers will implement a different file copy action/API call in a later release.
Yes, in this thread, among other places.
Funny, yes you are right. OK, admit, it's a very long standing issue! 
rcoleman1943 are you serious?
It's the very most basic function in a file manager.
EVERYONE manages their files over network these days. Mostly from a NAS. And a majority of people use Windows.
This effect almost every user who buy and use this product.
I asked for fixes. I do want to use the product. But it must be a working product for me to buy it don't you agree?
Whats the use of your post? What did you bring to the issue?
My question is sadly valid. The product cannot handle samba/windows shares since atleast 2006.
It doesn’t affect everyone. I can copy files at full speed over my LAN using Opus.
And we’ve said we are going to make the changes you want, so there’s no point arguing about this.
I just purchased the Opus Pro today and noticed this issue as well 
. Looking forward to the fix as I have a 10g connection to my NAS. Love the software besides this issue. Worth the money for sure!
You sadly will have to wait years, they have promised to fix it over 4 years. So dont get your hope up.
Thanks for the update. Please don't de-prioritize this because ZyntaX is being a d!*k
Any day now 
I am seeing this same behavior using DOpus 12.31 on Win11. Explorer copies at ~45MBps vs DOpus at ~2MBps to the same NAS.