I am configuring QNAP NAS TVS-682 these days and it came with i3 7100 @3.9GHz (2 cores, 3 threads) CPU and 8GB RAM which can be realistically used as a PC (virtual machine). For starter I have only a single 8TB 3.5" Seagate IronWolf HDD and 500GB Samsung EVO SSD for write/read caching. I hear that some folks upgraded it to i7 CPU and 64GB RAM and you can also install add-on card with two 10 Gbps ports.
It has Container station that uses LSX and Docker type virtualization for Linux VMs and QNAP's own (I think) KVM virualization (for Windows, Linux, Unix, Android)
Not sure about hosting mir server on it, didn't try it yet even locally. In any case it is all about the CPU & RAM and most NAS boxes out there emphasize storage size and speed of HDDs rather than processing power, and those that have good CPU and everything else that matters can get quite expensive.