Guest Additions
Guest Additions is something you can install inside your virtual machine after a guest operating system has been installed. Doing this will improve video support, add mouse integration, shared folders, time sync, shared clipboard and several other features.
Install Guest Additions by clicking Devices — Install Guest Additions
Each new version of Virtualbox brings a corresponding new version of Guest Additions so if you move to a new version, install the new Guest Additions.
RAM Memory
You can assign as much RAM as you want but don’t forget to leave plenty for the host machine. In my experience a few GB is plenty.
Processor(s)
My new 64-bit machine has quite a bit more processing power than my last one and I’ve found this has made the biggest difference for me. Inside of your virtual machine’s settings click on System and then the Processor tab. Here you can assign multiple cores to your virtual machine.
After doing so be sure to turn on IO APIC. You can find a checkbox for this back on the Motherboard tab
Assigning one quad core processor (shows as 4 cpus in vb) to my Ubuntu 12.04 virtual machine fixed any slowness issues I was having. Any RAM or Video changes I made didn’t make much of an impact.