Getting ~1.5Gbit iperf3 speeds versus expected ~2.3Gbit between DS918 and M1 MacBook Pro

**tldr:** I am getting differing results (\~1GBit less) from iperf3 using identical adapters and the same ethernet cable on Windows 10/MacOS to my DS918+, ***unless I use the "-p 2" flags on MacOS***. When I run iperf3 without flags between Win10/MacOS I get expected \~2.3Gbit results.

Win10 client - DS918 server - \~2.3Gbit

MacOS client - DS918 server - **\~1.5Gbit**

Win10 Client - MacOS server - \~2.3Gbit

What is going on here?

**Original Post:**

So I finally dished out for a QNAP QNA-UC5G1T 5GBE adapter after spending a long time trying to unsuccessfully get a pair of two Killer 3100U 2.5GBE adapters to work with my 918+ to m1 MacBook Pro. Unlike the Killer adapter, this installation went relatively easy except that I didn't have to install the driver a second time per the instructions and have no idea why.

I was expecting at least 2x performance given there is still a 2.5GBE adapter connected at the other end to the MacBook Pro, but for some reason I'm only achieving roughly \~1.55Gbit/sec . This is my first time using any kind of multi-GB adapters so I'm assuming it would run at roughly the same speed (2.29GBbits/sec) that bb-qq achieved when performing [his performance test of his driver](https://github.com/bb-qq/r8152) at the bottom of the RTL8156 (2.5GBE) driver page.

I installed iperf3 natively from the synocommunity repository and running it gave me the following:

iperf3 -c 192.167.1.200 - - - - - - - - - - - - - - - - - - - - - - - - - [ ID] Interval Transfer Bandwidth [ 4] 0.00-10.00 sec 1.82 GBytes 1.56 Gbits/sec sender [ 4] 0.00-10.00 sec 1.82 GBytes 1.56 Gbits/sec

***However, only when I run iperf3 on MacOS with the -c -P 2 flags, gives an expected \~2.3Gbits/sec speed.***

iperf3 -c 192.167.1.200 -P 2 [ ID] Interval Transfer Bandwidth [ 4] 0.00-10.00 sec 1.32 GBytes 1.14 Gbits/sec sender [ 4] 0.00-10.00 sec 1.32 GBytes 1.13 Gbits/sec receiver [ 6] 0.00-10.00 sec 1.31 GBytes 1.13 Gbits/sec sender [ 6] 0.00-10.00 sec 1.31 GBytes 1.13 Gbits/sec receiver [SUM] 0.00-10.00 sec 2.64 GBytes 2.26 Gbits/sec sender [SUM] 0.00-10.00 sec 2.63 GBytes 2.26 Gbits/sec

When I used iperf3 between the two 2.5GB adapters on Windows 10 and MacOS with no flags I get the expected \~2.3Gbit speed.

So here's the kicker- when I use an identical adapter on my windows 10 box, using iperf with no flags, I get the expected \~2.3Gbit speed.

So, what is going on here?