A very logical reason why Ubuntu runs slow on your system is the lack of resources. If you have a ton of applications running in the background, you can't be expecting VirtualBox to run smoothly. It needs some resources and when you distribute your CPU, GPU, RAM to other apps as well, VirtualBox is left with scarce resources.
Talking about resources, when you create the VM, you allocate RAMs, CPUs, etc to it. Your VM tries to adjust its performance as per the resources you allocate to it, so, if you feed it fewer resources, it will make sure that it doesn't crash by dialing down the resources.
There are some other settings that we are going to make to make the app work. If you want a smooth run with Ubuntu, then try the solutions and workarounds mentioned hereinafter. Without wasting any time, let us hop into it.