Question i7 8750H Flatlines at 100% when downloading 100Mbps+

[Gamefreaknet]

I have Gigabit Ethernet at my place however my Alienware M17 (Alienware 03R2RY, Intel HM370) currently now 6/7 years old seems to bottleneck (or rather the CPU just instantly spikes and sits at 100% until the download is over).
My SSD most of the time whilst downloading sits at 5% - 20% usage (rare spikes of 50%+) and CrystalDiskInfo Reports that both my SSDs are Healthy, my boot SSD [NVMe] 99% Health, my spare SSD [SATA 6] 100% Health.
During the downloads my CPU will tend to drop from 3.9Ghz to 3.4Ghz (longer/larger downloads it will slowly drop closer to 3.2/3.1Ghz).
Full Specs:
i7 8750H - with UHD 630
32GB RAM (2x 16GB 3200mhz)
3TB Total SSD storage (2TB NVMe, 1TB SATA6)
RTX 2070MQ
Killer Wireless AC1550 with E2500 Gigabit Controller (I always purely go for Ethernet over wireless)

 

[Ralston18]

PSU: make, model, wattage, age, condition (original to build, new, used, refurbished, used)?

This:

"Killer Wireless AC1550 with E2500 Gigabit Controller (I always purely go for Ethernet over wireless)"

Does the problem occur if you use/test a wired connection?

Look in Task Manager and Resource Monitor - either one or both tools may reveal what is using up CPU % - especially during downloads.

 

[Gamefreaknet]

PSU: make, model, wattage, age, condition (original to build, new, used, refurbished, used)?

This:

"Killer Wireless AC1550 with E2500 Gigabit Controller (I always purely go for Ethernet over wireless)"

Does the problem occur if you use/test a wired connection?

Look in Task Manager and Resource Monitor - either one or both tools may reveal what is using up CPU % - especially during downloads.

It's a Laptop not a desktop. I am always on Ethernet (I rarely swap to using wireless).
Had a look in task manager processes and looked like my Antivirus (Avast One Essential) was using 20% - 40% CPU. Probably going to Uninstall as a lot of posts on Reddit claim it might by CPU mining or similar...
The only other processes using high CPU were:
System
Firefox (around 30% usage but I've tested multiple browsers on my PC before and Firefox has been the least CPU intensive).

Regarding my Ethernet it is purely using cables marked as CAT8E Rated buuut I do know a lot of cables marked as CAT8/8E aren't actually CAT8/8E

 

[Ralston18]

Keep looking - remember to check Resource Monitor.

There also Process Explorer (Microsoft, free).

https://learn.microsoft.com/en-us/sysinternals/downloads/process-explorer

The objective being to compare system performance when there is no apparent bottlenecking and then again when bottlenecking occurs.

Look for differences.

And change the Ethernet cable. Cat5e, round UTP, all copper, 22-24 AWG.