Rsyncing via LAN deadly slow

[Stephane Ascoet]

Hi, I'm backing up contents of my 
GOBook(http://sascoet.mutu.fdn.fr/monalbum/albums/sa-12082010fujitsugigamos2300scsifleury-les-aubraisrecusdemonsauveurcomputermagneticsdmc-lx2/sa-1208201012h39fujitsugigamo2300scsiconnecteagobookfleury-les-aubraisrecusdemonsauveurcomputermagneticsdmc-lx2.jpg) 
before it dies completely on a dell laptop via my LAN(netstat indicates a 
11.8MB/s speed between the two computers via this one).

On the GOBook I launch "rsync --daemon"(3.1.1-3 version)

On the dell: rsync -abSbHXz --progress(3.1.2-1 version)

CPUs are doing almost nothing. So does the GOBook HDD. The destination is on a 
mounted logical volume in Ext2 on a nvme SSD.

And the indicated speed transfer is around 50kB/s!!! I tried -W option: it's 
even slower. Same thing when I put off the "z"(anyway, CPUs aren't doing nothing).

If I netstat during the backup, it appears that there is about 4MB/s used, so 
almost 100 times more than the actual data written on destination! And why not 
11MB/s since my network can do it?

I spent hours searching an explanation on the Web, nothing explaining what's 
happening in a sufficient satisfaying way for me. Does somenone on this list 
have a clue?

-- 
Sincerely, Stephane Ascoet