2400 Mbps
I was very close to the router. Like no more than 2 meters.
It starts at a reasonable speed, but then it reduce a lot. It seems there is a buffer saturating quickly.
Resolved ! !!
Hello. I'm the original poster...
I dont understand anything in what you say, but after reading those posts, I formatted the HD in EXT4, and now it's working seamlessly on Mac. It might not be the fastest, and I would've preferred EXFAT, but at least it works as intended with all my Macs, even Apple Sillicon.
I suggest making this clear in the instructions to save others from wasting time and effort like I did. EXT4 isn't integrated into the system, so formatting directly is impossible. However, there's a free and simple way : I used the demo of Paragon EXTFS for Mac: Write/read access to Linux files under macOS High Sierra - extFS by Paragon Software for Mac
Sorry if my input seems obvious to you, but as a non-IT person, I think it could help someone like me. I didn't find it clearly stated anywhere, despite years of frustration and this question on the support forum.
Thanks to @dchao and @admon for guiding me with your initial reply discussions!
For me, this topic is now resolved.