Why it is better to use only 1 processor core in virtual machine?

Don't quote me but I think it's due to vm requiring a core to be free to perform its duty. So if you assign all cores it'll spend more time waiting for the cores to be free.

 

That's not my experience. I've been running Virtualbox on the i7 CPU for several years and assign at least 2 processors to each VM, whether Windows 7 or Ubuntu. I find the VMs are more responsive with little to no impact to the host.

 

Why not? There's no risk in playing around with VM settings. Tweak processor count and memory to find out what gives you the best balance between your VM and host machine performance. Try various settings to see which you like best.

There are no optimal settings in general. It comes down to what you do with your VM, how responsive you would like it to be, how responsive you would like you host machine to perform while the VM is operating.

For instance, if all you do is dink around with VM and do no serious work on it, then you may as well config it with minimal resources to lessen the impact to the host machine.

 

My guess is that the optimal settings depend on what you want to do. For my particular case, they are 1 core and 4GB of RAM. I use Windows 7, 32bit (no point in assigning more than 4GB) and the programs I use do not care for more cores.
I run VMs in a mid-2012 i7 MBP with 16GB of RAM and a 500 GB SSD. FWIW, when I installed the SSD and increased the RAM to 16GB, VM performance improved significantly; much more noticeable that any settings change.