@AiDiN sadly there's people out there that will take for granted what you are saying, I know it was probably just lot of fun for you.. but just don't, unless you end your phrase with a "just kidding".
Now returning to the issue.. there is no issue:
1) SnapDragon 810 - 4 cores for regular tasks @ up to 1.5 ghz, 4 cores for high performance @ up to 2.0 ghz.
2) Android is based upon Linux. Like all Linux and Unix systems you need to understand the difference between CPU usage and Load. The fact that the CPU is at 100% doesn't mean that the load is at 8, more on this below.
To understand how the cpu usage is distributed is complicated and it's not something that a simple APP can really simulate, in fact did you ever read the statement to never believe a benchmark score? It's real life and real usage that dictate the performance and usage.
Now, to let you understand how the cores are awaken during regular usage of your phone:
So we said that the SD810 has 8 cores, 4 low/mid performance and 4 high performance.
Now Imagine that 4 low/mid performance cores are 4 city roads and 4 high performance cores are 4 highway lanes. The following example will let you understand when the cores are going to be activated, mind also that the system/OS may use multiple cores also if are not necessary. This is done by the governor in order to keep the system smooth. A power efficient governor will use just one core until the load require another core to be activated but the device may become sluggish. (Think about Ultra Stamina Mode)..
Example 1: Load: 0.9 - 0.00 - mostyl dozing
Road 1: 4 cars
Road 2: 3 cars
Road 3: 2 cars
Road 4: 0 cars - road closed
HWY 1: 0 - lane closed
HWY 2: 0 - lane closed
HWY 3: 0 - lane closed
HWY 4: 0 - lane closed
Meaning that:
Core 1 has 4 processes to handle, it's load is at 0.4 - No need to use another core,
Core 2 has 3 processes to handle, it's load is at 0.3 - No need to use another core,
Core 3 has 2 processes to handle, it's load is at 0.2 - No need to use another core,
Core 4 has 0 processes to handle, it's load is at 0.3 - This is waiting for more regular tasks.
The Highway lanes are all closed. - no need to use those.
Example 2: Load: 2.2 - 0.00 - Regular usage, screen on, some social networking and chrome browser
Road 1: 8 cars
Road 2: 10 cars
Road 3: 10 cars
Road 4: 2 cars
HWY 1: 0 - lane closed
HWY 2: 0 - lane closed
HWY 3: 0 - lane closed
HWY 4: 0 - lane closed
Meaning that:
Core 1 has 8 processes to handle, it's load is at 0.8 - Able to get 2 more tasks,
Core 2 has 10 processes to handle, it's load is at 1.0 - Need to send a process to another core,
Core 3 has 10 processes to handle, it's load is at 1.0 - Need to send a process to another core,
Core 4 has 2 processes to handle, it's load is at 0.2 - Able to get 8 more tasks.
The Highway lanes are all closed. - no need to use those.
Example 3: Load: 4 - 4 (8) - Lot of background apps, listening music via spotify, on a call with a mate, playing a very detailed video game - plus regular background tasks - Phone here will probably lag!
Road 1: 10 cars
Road 2: 10 cars
Road 3: 10 cars
Road 4: 10 cars
HWY 1: 10 - lane in use
HWY 2: 10 - lane in use
HWY 3: 10 - lane in use
HWY 4: 10 - lane in use
Meaning that:
Core 1 has 10 processes to handle, it's load is at 1.0 - Need to send a process to another core,
Core 2 has 10 processes to handle, it's load is at 1.0 - Need to send a process to another core,
Core 3 has 10 processes to handle, it's load is at 1.0 - Need to send a process to another core,
Core 4 has 2 processes to handle, it's load is at 1.0 - No more tasks can be executed at once. - LAG
The Highway lanes are all open and in use. - unable to execute more tasks. - LAG
In few words, it's the load or the number of processes running that dictate when to use more cores or less, not the cpu usage. Do not believe apps and benchmarks but of course, if you want your phone to have all cores on to do simple day by day usage, let's say 1 process per core no matter what.. you can do that but mind that involve:
1) Rooting,
2) Modifying governors or Installing a different one,
3) accepting a possible 6 hours battery life total.
No one want that.
TL/DR: Your phone works just fine.
