How to allocate more processor power to my (Ubuntu-based) Virtualbox system in Windows 7?

Getting a host computer that's running something better than an i3 would be the real answer (IMO). :)

Regardless of that...

The reason it's using 25% at most is because your i3 has 2 cores, but 4 threads, so the host sees it as 4 processors.

The VM is set to use only one of them, up to 100%, so when it's maxed out it's using only 25% of the total CPU power of the host.

For setting up VB to allow more than 1 virtual CPU, additional settings need to be configured:

• enable I/O APIC in the Motherboard settings tab

  

• enable hardware virtualization (VT-x/AMD-V) in the Acceleration tab

  

To avoid problems, don't set it any higher than your number of PHYSICAL cores; so in your case, don't try to use more than 2 virtual CPUs.

You should not configure virtual machines to use more CPU cores than you have available physically (real cores, no hyperthreads).

• Quote source
• Additional settings source

If the Acceleration options are not available, ensure you have enabled virtualization technology in your BIOS.