1: we can not correctly answer this. It is not only affected by your system but also by the amount of resources you will make available for the VM. A VM will cut down your resources but how much you will need to find out the hard way. Then there is the game itself. Some games work very will with a VM others are impossible to play. In general it is best to stick with the main OS for games. Games are resource hungry anyways.
2: So as explained with 1: My advice would be Windows with Ubuntu in VM. Games run better on the main OS. VM's will always cut into your resources.
A possible setup for trial purposes would be to install an OS and then install both Windows and Ubuntu and try if you can find a sweet spot where games run as good as you want in either of the VMs or if the main OS still is the best option.
The best security is done with a router (if talking about people trying getting access to your machine) and not with your OS so what OS is main and what OS is in a VM does not really matter then. Secondly comes common sense: do not visit shady websites (when talking about virusses and malicious programs).
Depending on the games you might also be able to use Ubuntu with wine. I play Guild Wars 2 using Ubuntu+wine. No guarantee all games work and most times it is a bit of work to get it working well but it might be an option.
In my own experience GW2 is about 10, 20% less good than when I play it on a Windows system. But this is not speed related but graphics related (graphics are a bit less).