P2421DC, USB 3.1 HUB problem

@duracer  sorry, when I wrote that first reply I missed the fact that you've also experienced this behavior on other displays, including non-Dell displays.  In that case the explanation I just provided might still be what's happening, but the issue may be on the system side.  If you haven't already, maybe check the BIOS configuration of your system to make sure that simultaneous USB 3.x and video isn't disabled on the system side.  Technically it's possible for a USB-C port to only support USB 2.0 at a hardware level since USB 3.x is an optional capability of a USB-C port (so is video output), but the T480s supports Thunderbolt 3, so I doubt it's an actual hardware limitation.  But if you're seeing 40 MB/s (megaBYTES), then that is 320 Mbps (megaBITS), which sounds about right for the real-world throughput of USB 2.0 (480 Mbps or 60 MB/s).

Also, have you verified that the USB peripheral itself isn't the bottleneck?  You didn't specify what USB 3.x device you're actually testing with, and you also didn't specify what type of performance you see when connecting that device directly to the T480s.  Those are important data points.

But in any case, if you're seeing this issue with a Lenovo system even when using other non-Dell displays, this might be a question better asked on the Lenovo Forums.

@duracer  I forgot to address the question in the last part of your post.  With the P2421DC, you don't have a way to run dual QHD and USB 3.x at the same time if you also want to be able to charge your system from the display.  If you were willing to give up being able to charge your system, you could use a USB-C to DisplayPort cable to connect to the display for video output, and then use a USB-C to USB-A cable to connect the display's USB-C port to your laptop's USB port in order to get USB 3.x data.  That would give you enough video bandwidth to run dual QHD in a daisy chain and USB 3.x data, but the downside of course is that you lose system charging this way.  Other than that, some older Dell USB-C displays included an "upsteam" USB 3.0 port that could be used to get USB 3.0 data back to the system.  In that case, you could use the USB-C input ONLY for video and charging, and then you'd connect a regular USB 3.0 cable from the display to your laptop in order to carry USB 3.x data -- but the P2421DC seems to have eliminated that option.  On the P2421DC, the only way to get USB 3.x data out of that display is through its USB-C port, and unfortunately, when you're using a system that only supports DisplayPort HBR2 over USB-C, running USB 3.x data over USB-C means you won't have enough video bandwidth available for a dual QHD daisy chain.  If you had a system that supported DisplayPort HBR3 over USB-C, you would have enough bandwidth for dual QHD and USB 3.x because HBR3 can carry more bandwidth per high speed lane, but DisplayPort HBR3 systems are still fairly uncommon today.

So yes, with your current system, the only way to run dual QHD and USB 3.x over the same cable would be with a solution that involved Thunderbolt 3, either a Thunderbolt 3 dock or a Thunderbolt 3 display that included a daisy chain output.  However, there aren't many Thunderbolt 3 displays out there, and there are even fewer that have a daisy chain output, and even among the ones that do, it's typically a Thunderbolt-only output, which means the SECOND display in the daisy chain would ALSO need to be a Thunderbolt 3 display.  The Thunderbolt 3 displays I've seen do NOT support connecting a regular USB-C display to their outputs.

The DisplayLink adapter option you suggested would also work, but DisplayLink has some drawbacks compared to running displays via native GPU output.  But if you've seen exhaustive posts of mine, you've probably also found the one where I wrote about that in detail.

Lastly, just as a general tip, when posting questions about technical issues like this, it's helpful to provide all of the relevant technical information.  In this case, you only mentioned a single display even though you were apparently running a daisy chain setup.  That would have been worthwhile information to provide upfront.  I just happened to think about MST as a possible factor, but if I or someone else had NOT thought to ask about that, you and whoever was helping you could have ended up wasting a lot of time looking at the wrong things trying to diagnose this issue.  I understand that you might not have realized that having a second display attached via daisy chain would be relevant here, but in those cases it's best to provide additional information just to play it safe.  You never know what might turn out to be the crucial bit of information that helped solve the problem.

Hi @jphughan!

Thanks for your exhaustive posts. You are right in your first post. I had MST or daisy-chaining enabled on the monitors.

I am very surprised and disappointed about that. I thought the DP and USB have separated lines and pins in USB-C and are independent of each other. :0(

Therefore If I want to use only one cable without docking station I have to wait for monitors with Thunderbolt 3 or to connect the second monitor to the first via USB 3.1 and DisplayLink adapter.

@duracer  well if you're up for another exhaustive post of mine, I explained the various operating modes of USB-C/TB3 connectors with particular focus on the impact to display setups over here.  But if you don't want to read that whole thing, a USB-C connector has pins dedicated purely to USB 2.0.  Past that, it has four high speed lanes.  Each of those can be used for either USB 3.x, DisplayPort, or Thunderbolt 3.  When DisplayPort and USB 3.x are being used simultaneously, they each have lanes allocated to them.  No single lane carries both types of traffic at the same time.  The downside to this is that when you allocate lanes statically to either DP or USB 3.x that way, it means that if you're not actually USING very much USB 3.x data for example, the bandwidth available on those lanes is being wasted and cannot be used for additional DisplayPort bandwidth.  By comparison, Thunderbolt 3 uses all four lanes to carry a multiplexed signal of PCIe+DP, and the composition of that traffic can be dynamically altered by the Thunderbolt 3 controller as needed.  And USB4 will be adopting that model in order to make more efficient and flexible use of total available bandwidth rather than using static lane allocation.