Virtualizing older Mac OS versions—why not max out virtual CPU and RAM settings?

[iPodShuffle]

I know some people virtualize older OSes with the corresponding hardware of the time order to simulate a specific computer model. In my case, I'm hoping to virtualize bootable backups of old Macs of mine (as well as play around on fresh installations of older OS versions) and I don't care to preserve the experience of the slower hardware specifications of the time but would rather take advantage of my current Mac's power.

If I'm not mistaken, when selecting the OS version being installed in VMware Fusion, the settings default to a virtual hardware configuration that more or less match that time period, right? Is there any reason I shouldn't just pump up the CPU cores, RAM, et cetera, to the maximum allowable in the VM settings in order to get the fastest virtualization experience?

Does VMware Fusion have a dynamic "Maximum available" VM hardware setting that can either assign the maximum without me needing to specific select it, and/or dynamically adjust based on the available resources (which might change as I open and close applications on my actual computer)? Thank you!

[scott28tt]

You’ll starve your host of resources if you maximise those given to the VM, a better approach is to right-size a VM based on the resources it requires.

[ColoradoMarmot]

Two things, selecting the version of the operating system provides compatibility, not performance limitations.  Just as newer hardware can't run old OS's, you have to select compatible virtual hardware.  That's independent of performance, which a handful of special exceptions (e.g. DOS requires a system clock that matches what the OS expects).

As for resources, the rule of thumb is to never assign more than N-1 cores to the VM, where N is the number of physical (not hyperthreaded) cores.    Likewise, you always want to leave at least 4GB of RAM for the host OS.  If you exceed those limits, you'll starve the host for cycles (as Scott said) and actually slow down the whole system.  If you're running other things on the host, or are running multiple VM's at once, then you'll want to lower those limits.

As for dynamic, that wouldn't work because just as you can't insert and remove RAM or CPU cores while an OS is running on hardware, you can't do it virtually either.