I'm quite aware about how multicore CPUs affect apps that weren't built for them.
The PC I'm using has a 3.5 GHz clock rate and can turbo boost to up to 4.2 GHz. I remember playing this game on a 400MHz Celeron. I'm quite certain my CPU would have no problem with Red Alert 2 of all games, especially since an i7 should be way faster per Hertz than a Celeron that was released alongside the Pentium 3. Also, I'm fairly confident Windows is capable of moving any other process to the remaining 7 (logical) threads should the game require more resources.
Also, this particular game does not support resolutions above 1024x768. Even though I'm playing on a 4k screen, the game won't render at a higher resolution. The graphics are blurry from the scaling. Besides, the game runs constantly slow, regardless of how many objects are visible at the same time.
No, my PC is definitely more than fast enough for the game. If anything, I'd expect the game to be running so fast it becomes unplayable, but not the other way around.
I suspect it's simply some incompatibility with the OS. The game is most likely trying to make some calls that Windows 10 either doesn't recognize or simply no longer supports, which throws tons of exceptions around that the game needs to catch. That would explain why it's running so slowly.
But I can't really tell for certain without seeing the code, and frankly, I'm not really interested in looking at old code. I've got more than enough of old code to look at, at work.