That's a pretty old machine - does it use a Fusion drive, or have a spinning disk? If so, move the VM to a dedicated SSD - spinning disks will run slow, and Fusion drives don't play well with Fusion (no pun intended). Also, that's a quad-core machine, not 8 core. Virtual cores do not count. So you're configured with the guest at the maximum we'd recommend with a more recent MacOS version.
Next make sure you disable hyperthreading support in the guest, that slows VM's down considerably. Also remember, that MacOS guests have serious compatibility limitations - no 3d support for example, that cause performance issues.
Honestly, that machine is getting to be pretty marginal for running virtual machines, especially relatively modern MacOS on the host and MacOS guests (both the guest and host want 2 cores/4GB minimum, and that's just the OS - nothing else running). If the above steps don't help, it's probably time to upgrade hardware - just note that if you do, M1 machines cannot run x86 guests (and likely never will). You'd need to run an ARM based guest OS. FYI the current tech preview doesn't support OSX guests anyway, and when it does I suspect it'll be Ventura or later only.
