I am using 10Gbase-SR, and tried 40Gbase-SR4, TB adapter is JHL7440.
10G works very well.
40G can only link at ~20Gbps, 'cause AMD USB4/TB4 has only 1 data lane. and it is 10Gx4 configuration with 8 fiberoptic cores, thus at least 2 parallel sessions can reach 20Gbps.
The JHL7440 is the limiting factor.
AMD’s USB4 config carries 40 Gbps in both directions simultaneously (2x 20 Gbps lanes each way, although they’re bonded together so they behave like a single 40 Gbps link).
There is overhead that means it cannot reach the full 40 Gbps, but still much faster than 20 Gbps (likely around 30 Gbps).
However the JHL7440 can’t handle that. A newer controller such as the ASM2464PD or something from the Barlow Ridge controller family could reach higher speeds.
Out of curiosity, may I ask what NICs and dock exactly you are using? Thx!
Precisely.
Have a look at this table on egpu.io: the rows in question are 64Gbps-USB4v1 and 32Gbps-USB4v1. Although the data comes from eGPU memory copy measurements, I strongly believe that it will be virtually the same for NICs and you can expect ~8Gbps improvement if you switch to a dock like ADT-Link UT3G/UT4G with ASM2464PD(X) chip.
NICs
My dock is made by unknown vendor in china. but it works pretty well, and even has 60W PD supply. way better than crappy thinkpad docks.
My ASM2464 M.2 enclosure not working well with samsung SSDs, but quite ok with other brands. I can fio to 3.5GBps throughput, with 2 or more parallel sessions.
Thanks!
update: both google and duckduckgo say “no results for MCX354-FCTB”: is there a typo there maybe?
update2: found it, it should be MCX354-FCBT 
Man, now you made me really curious: may I ask for a photo, pretty please?
?
I’m quite bamboozled here: ASM2464 is an USB controller: why would you plug it into M.2 and then plug an SSD via USB into it instead of connecting an SSD directly to M.2? Am I missing something?
@Morgwai
I have an ASM2464 M.2 enclosure.
It can run in two modes:
ah, so an ASM2464 is on the enclosure side that provides an M.2 slot for an SSD, NOT like an ASM2464 on an M.2 card inserted into laptop’s M.2 slot as I thought before… Stupid me, I get it now
came with a sheet steel frame, with mount place for SFX PSU.
uh, my bad
ASMedia Technology Inc. Device [1b21:2463]
it’s a Zike Z666 (silly model number), 2464? 2463? ?? 
wait, are there any ASM2464PD products work with GPU/NIC whatever?
JMT ADT-UT3G $188? this is robbery!
This aliexpress listing is between 115-145USD depending on your location: that’s a bit cheaper, but yeah: USB4/TB4 chips are expensive.
I like the big red “launch-the-nukes” button 
There could be a reason that thing has 8 lanes of pcie3, there may just be overhead.