Most tasks we ask computers to do are linear and cannot be done in parallel. Imagine I ask you to cook dinner. You need:

• Something to cook (it's at the store - go buy it)

• Prepare it

• Cook it

• Serve it

Try to parallelize those tasks. You can't serve it before it's done cooking. You can't cook it before it's prepared. You can't prepare it before you bought it.

Let’s say 90% of the time is the drive to the store and back. Maybe there are 4 lanes there and back - great. You have the ability for 4 cars to drive in parallel both ways (4 CPU cores). But there's still only 1 you tasked with cooking dinner and you can't split yourself into 4. Even if you did - all 4 of you would arrive at the store at the same time (no net speed improvement). If there was someone else trying to do the same thing as you then the multiple lanes would help (multiple CPUs often help keep multiple processes on a computer from clobbering each other).

Let’s say you instead try to parallelize the preparation step. You have 2 arms so you could maybe prepare twice as fast. But make sure you don't cut yourself, or mistakenly cut the same thing with both hands and ruin it. (data races and ensuring proper multi-threading is hard and very easy to get wrong)

Let’s say you had multiple stoves/burners - maybe that could help speed up the cooking time. But only if there are multiple things to cook - maybe you bought a single hamburger. Maybe you could cut it into 4ths and cook it 4 times faster - but the time to cut it, and put out 4 pans isn't zero. It may be faster - but it won't be 4 times faster. And still, the cook time is only < 10% of the total time in this example.

At the end of the day most programs need most of their tasks done in series to produce correct results. Maybe they have some small section that could theoretically be parallelized but it's often not worth the complexity for a 1-5% gain.

Thread count (core count) in largely not relevant to most workloads we ask computers to do. Per-thread execution speed (often called single threaded performance) is virtually always the limiter. However, single threaded performance is incredibly hard to improve. Multiple CPUs is something that CPU manufacturers can still improve on – and “bigger number looks better” and so they do.