Yes, you definitely have "plenty" of computer horsepower, including RAM.
I am also getting some slowness or sluggishness from VM's with some experimenting today. My system is 10 years old (2009) and has a 4-core AMD CPU and 8GB of RAM. I'm using Linux Mint 19.1 Cinnamon and VirtualBox... and today I installed Fedora 30 and Windows 10 (October update). My BIOS has "Virtualization Technology" enabled.
The installs went okay, but I gave both VM's twice the RAM they requested, so 4GB for WIn 10 instead of 2GB, and 2GB for Fedora instead on 1GB. I later found that each VM was only given 1 CPU core, so I increased each one to 2 cores. This improved it some, but it still not really as good as it should be. The Linux Mint host system runs just fine with the reduced RAM and CPU cores. Your setup might improve greatly to give each VM 4 cores of your CPU, if you haven't tried that already.
This link gives a number of tips to help with speed issues, including those I already mentioned above. Another tip in the article that I always use to help with speed is to create "fixed size" virtual disks instead of dynamically allocated disks.
Updating each VM might give some improvement with an updated version of VirtualBox Guest Additions. I allowed Fedora to update but not Windows 10 (that's a big reason why I don't run Windows 10!). I can't say if Fedora improved or not though. Actually, for me, it runs mostly well enough to be acceptable since I don't need/use a VM in most situations. But if I wanted to run the VM more then the slowness might bother me much more than it does now.
Here are a couple more links (
here and
here) that also echo the tips in the link above, but maybe offer some different approaches that you might want to try. You may also find more and better info with some heavy Googling around, but the real time consuming part is testing and trying new things, rebooting the VM, maybe reinstalling the VM, as you search for what works best for you.
Cheers