As you may, or may not know, 4 core hyperthreading CPUs show up to the system as 8 cores. The advantage of HT is supposed to be that the system can use the additional "cores" created by hyperthreading, or other physical cores, for background tasks, other applications, etc., which then frees the core being utilized for the application to complete it's task rather than having to share it's resources with another task.
As to why your cores won't utilize more than 12.5% of the CPU, that's basic math. 8 cores times 12.5% obviously equals 100%. The more important question is whether or not each core is using it's full share of resources, which, by default, it should be. Disabling HT probably isn't going to gain you much if anything, but it really depends on how the application or game, in question, is written. There are buggy ass games like Farcry 4 for example that most people find won't run at all on dual core cpus (So forget about using the G3258 with it) and requires a 4 core CPU, but then refuses to utilize more than a single core for gameplay.
If the game you're trying to serve is not written to utilize hyperthreading or multiple cores, then you might be better off disabling hyperthreading, at least to see if the additional 12.5% of resource allocation is enough to resolve your issue. In reality, this is a problem with the game or application, not the CPU, but that may work as a work around.
I'd also make sure that the Turbo boost feature is enabled in the BIOS to allow for the natural boosting of the cores under demand.