Are you lagging? if not who cares.
There are two non-issue reasons why this is happening, Low resolution (why run BF4 in a TINY window?), offscreen window (80% of LoL is offscreen, I wouldn't be surprised if its not bothering to render half of the stuff that you can't see).
The other more important issue is CPU core load, I'm not familiar with LoL but from what I've heard LoL was a rushed job (Indie start-ups tend to cut corners) so most likely only single threaded and thus can only use one core, 15-30% would probably indicate you have a 4 core system no doubt, and finally LoL has very simple graphics leading to low system requirements.
Check you temps, and cooling, and try some overclocking (of your CPU), or buy a new CPU, run your game at more than 480x640? to actually stress your GPU, lock your affinity so turbo-boost can work better, I use RadeonPro to lock affinity and the OSD for testing GPU load in fullscreen, maybe theres an nVidia option somewhere.