12-15% is about the percentage you'll get when an application only uses 1 out of 8 cores on a i7 (4 cores + hyperthread), so it does seem like your applications are only using one core.
Most file hashings will only use 1 core to avoid overloading the disks. Many file and video compression softwares use multiple cores, but not all - what softwares are you using for converting and compressing?
As for the 0%, it's possible if few background processes are active. It could be very well oscillating between 0% and 1% when idle, thus showing 0 frequently.
I'm not seeing a problem with your situation, other than you are not using applications that support multiple cores where possible/applicable.
If anything, you should be able to run 2 of those tasks at the same time without any speed reduction, and a single task won't impair your ability to use the computer without hiccups.