Did you mean an i5-8400 instead? As there is no i5-8700 - that’s an i7, which would have the same number of cores as the 9700.
The i5-8400 would be fine for most IT homelabs; there are some specific applications that really need more power than that (especially any of the Cisco bits from what I’ve heard, but I have no first-hand experience with those).
What exactly are you planning on doing with the homelab?
Out of curiosity, do you have everything in /tank? As in tens of thousands of directories or files directly in the root versus a tree of directories?
You don’t have to run the emulators on your NAS. In fact, you probably shouldn’t since your NAS is unlikely to be plugged in to any video output.
Pretty much anything can just point at your NAS’s shares for access to any and all emulation. Heck, I think an Amazon Fire Stick can even do it.
If your buddies want to bring over Shield Pros to play, all you should need to do is let them on to your WiFi and have read access to your roms or ISOs directory.
So… I guess I’m not quite sure where you’re going with the question?
Where are you seeing performance issues? As your issue might be less with your CPU and more with storage or some other bottleneck.