I posted a recommendation on your other thread concerning the slowdowns you've been experiencing, if you haven't returned your motherboard yet, you may wish to try it. As @tkaiser notes, a bit of troubleshooting is a good idea.
Concerning a new motherboard though, as @TheLostSwede notes, without knowing your goals for OMV, its hard for people to make recommendations.
If you do decide to buy a new motherboard, a general suggestion is to find hardware that has good Linux support. OMV runs on Debian Linux, so Linux compatibility is essential.
When researching a particular motherboard, try an Internet search with its name plus "Linux" or "Debian", and a separate search with the processor's name and "Linux" or "Debian" just to see what problems may have been reported. For example, searching the ASRock board you currently have comes back with issues regarding C States and stalled network transfers. Luckily, there's also info on how to fix the issues, which I noted in the other thread. Files by samba very slow
Working with computers since the days when unboxing and set-up required 3 weeks with a soldering iron!
I guess I have got lucky with the desktop motherboards always working well with Linux but every real server motherboard should work fine with Linux. So, maybe look for a server grade motherboard if you can't find a desktop board compatible with Linux.