I think your problem is, at least in part, due to the fact that you're connecting via usb. No matter how fast your drive is capable of going, your machine has to negotiate the read/write speeds based on the number of lanes available for the entire system.

You can think of it like this: all of your usb ports share physical 'data lanes' that exist on your machines motherboard. These data lanes send information to and from your external device and the cpu. Additionally, most motherboard manufactuers hardwire various internal components into these data lanes as a way to save money without sacrificing hardware features. So now your external drive has to share a limited number of data lanes with all of your usb ports + anything else the manufacture decided to hardwire into.

When you connect your usb device to your machine, the device tells your operating system 'hey, I can do 100000 writes per second' then your operating system takes a look at all of the data lanes and determines how many lanes it can allocate to the external device, responding with 'ok. This system is very busy so I need you to do 200 writes per second instead of 100000'

Generally, when people talk about how fast nvme is, it's not because its just 'better' than everything else. It's because its usually connected directly to the motherboard via m.2 slots. These m.2 slots usually (but not always) have dedicated data lanes to the cpu.

I know this stuff can be confusing and manufactures make it worse with how they advertise their products but I hope this helps.