Ethernet throughput is HALF as fast on Apple Silicon Mini and Air vs. older intel MBP and iMac

I'm in the process of replacing a 10yr-old 27" intel Mac and an intel i7 MacBook Pro with a Mac mini M2 (2023) and a MacBookAir M1 (2020). And I'm finding that the measured ethernet throughput on both of these new Apple Silicon devices is HALF what is was on the older intel computers -- 600Mbps vs. 1.2Gbps.

Here's the situation:

• 1.25GBps uplink with 2.5Gbps unifi switches between the computer and WAN
• I'm using a 2.5Gbps USB ethernet dongle (Asus brand)
• When I plug it into my older MacBook Pro or iMac, turn wifi off, and use SpeedTest.net, I routinely get >1.2-1.3Gbps down.
• Now, if I plug that same USB ethernet dongle into either of the Apple Silicon computers referenced above, and use the same ethernet cables, same switches along the way, etc. my tests rarely top 600Mbps... even if I do this only 30 seconds after the MBP test.
• I've had the same experience with an external Startech 10Gbps Thunderbolt 4 ethernet adapter as well, and I've double-checked the speed on the Apple Silicon computers using wifiman.

Both Apple Silicon computers are running Sonoma 14.0 (latest version as of this morning). Older MBP is running Ventura 13.6. (The iMac was running a much older OS.)

Is there some configuration setting I'm missing? Or is Apple Silicon just really weak on Ethernet?

Thanks!