Tested on a new PC with a Cat5e ethernet cable, nothing else connected.
When I was on the 300 I always ran speed tests on my sons PS4 and always for frat speeds close to 300 now I’m on 900 and running the same speed test in PS4 and getting 100 max never over it
Does the PC have an Intel network card or something else like Realtek? Some cheap Realtek network chipsets have much lower throughput as they rely on the cpu much more than Intel network chipsets. You could try adding an Intel PCIe network card - ex-server pulls are quite cheap on the bay.
Realtek is fine for client devices where the difference in throughput often isn't a big deal and if you already have Realtek NICs then it's probably not worth replacing them.
The Intel NIC does offload CPU usage, but not by much - in total CPU usage, the difference is ~3.5%