2.5Gbps Ethernet RJ-45 to USB C Adapter poor performance

By far the easiest way to cause poor performance, instability, overheating and crashing is to install ANY third-party speeder-uppers, Cleaners, Optimizers, or Virus scanners, Bit Torrent, or a VPN that you installed yourself. 

¿are you running any of those?

is Ethernet TOPMOST in the list of Network devices?

I can't replicate things you can only detect or verify with third-party utilities like perf that require HomeBrew or other environments.

Actual Speed:

The good way to check the actual connection speed USED to be Network Utility, But in Catalina and later, Apple has deprecated Network Utility and now you have to use a Terminal command to see your actual connection speed. First, you need to know what en number the link is. then you use a command like this one, substituting the actual en number.

my main Ethernet connection uses BSD name en2 (as shown in) :

 menu > about this Mac > (system report) > network:

Aquantia AQC107-B0:

Name: ethernet

Type: Ethernet Controller

Bus: PCI

Slot: Slot-3

Vendor ID: 0x1d6a

Device ID: 0x87b1

Subsystem Vendor ID: 0x1d6a

Subsystem ID: 0x0001

Revision ID: 0x0002

Link Width: x4

BSD name: en2

Kext name: AppleEthernetAquantiaAqtion.kext

Location: /System/Library/Extensions/IONetworkingFamily.kext/Contents/PlugIns/AppleEthernetAquantiaAqtion.kext

Version: 1.0.64

Terminal command:

ifconfig en2 | grep media

with this as my output for 10 Gigabit Ethernet:

media: 10Gbase-T <full-duplex,flow-control>

For ‘regular’ Gigabit Ethernet, you should get this instead:

media: 1000baseT <full-duplex,flow-control>

Errors detected:

To see if an Ethernet link is throwing more than a handful of initial errors, you can use Terminal command:

netstat -I en2

This is the resulting output. Counters are In-packets, In-errors, Out-packets, Out-Errors, Collisions. There should never be more than handful of errors from starting up, and in most cases, NONE.

Name       Mtu   Network       Address            Ipkts Ierrs    Opkts Oerrs  Coll

en2   8163  <Link#4>    00:01:d2:1a:00:dd   696697     0   484301     0     0

en2   8163  grantsmacpr fe80:4::461:ea0d:   696697     -   484301     -     -

en2   8163  192.168.0/23  192.168.0.204     696697     -   484301     -     -

Reading the top line, If the link were running beyond its ability to run and be stable, for example it auto-speeded to 10Gb but the cabling could only reliably support 2.5Gb, we would see non-zero errors counts, and errors increasing over time. (and possibly, disconnecting)