Multi Core processor not using all cores

I have an Intel i7 8700 12 core processor and many of the cores don't appear to being used. there are times when the computer is slow to respond to changes (starting applications, change web pages etc.) Is there a way to utilise all the cores to speed thing up, or is the reference to 12 cores a bit misleading in terms of computing power?

Windows 10, Ver. 1909. Build 18363. 1139

Intel i7-8700 6 cores/12 threads.
It is more the potential use rather than actual use which depends on the particular application/task.
Applications have to be designed to use all those cores/threads.
e.g. a professional CAD application would probably make full use of all those cores/threads.

The examples you quote are I/O operations to/from your HDD/SSD.
In the case of the Browser caching of pages and pre-emptive downloading of pages.
In loading applications Prefetch and so on.
The number of cores/threads has very little impact on that. It is much more a hardware issue like an SSD being much faster than a mechanical HDD.

In your picture the task is so easy that the proccessor is not even turboing up to 4.6 GHz either.

What you are seeing is normal. It is partly the result of core parking and the normal way threads are scheduled.
Core parking is an energy saving feature of most modern multicore processors. When CPU usage is relatively light some cores (often half or more) will essentially be parked or disabled. A parked core consumes very little power. When CPU usage is high enough to warrant it parked cores will be enabled and made available for use. This happens very quickly.

Your CPU supports hyperthreading. It actually has 6 physical cores and 12 logical cores. The OS sees this as 12 cores. The logical cores are organized in pairs that share a lot of CPU components. Intel estimates this improves performance up to 30% for a compatible workload for about a 5% increase in CPU resources. Hyperthreading can be disabled in the BIOS in which case the OS would see only the physical cores, in this case 6. In some case this has a performance advantage but usually only with operating systems that are not hyperthreading aware. That would be XP and older.

It is difficult to write an application that really takes advantage of multiple cores and as a result most do not. For many applications using multiple cores isn't appropriate. Games traditionally did not really tale advantage of multiple cores. They would use them but there is a big difference between actually taking advantage of them. This is changing - slowly. Currently there are few home user workloads that can really take advantage of 6, let alone 12 cores.

Thread and core usage in a modern OS is very complex and often not intuitive. The goal is to optimize performance. Spreading the load among all cores might seem like a good thing but for the sake of performance it often isn't. When a thread is created it is assigned an "ideal core" which will be used whenever it is available. This tends to optimize performance. This tends to produce in the short term relatively high usage in some cores while other have little use.

Core use depends on the program and how it uses cores, as others have said.
I have a Ryzen 7 3800X with 8 cores, but 16 "logical processors".
When I run Folding@Home, all 16 "logical processors" are in use.
When I run older programs like WinLAME or DVDFlick, only ONE "logical processor" - or half of one core - gets used.

Here, as everywhere, comments are about the extent to which one program will exploit the hardware. Well, what about more than one?